Luka Doncic can appear to lack no superpower on the basketball court, where the 22-year-old Slovenian star regularly treats N.B.A. fans to long-distance floaters, nutmeg passes and playoff fireworks. But cyber sleuths have been flummoxed by the inconsistency he displays during a more pedestrian task: writing his name.

Many collectors believe that an elegant signature of Doncic’s name on the lone copy of a basketball card that sold for $4.6 million this year was written not by him, but by his mother. Like the signature seen on many of his other highly coveted trading cards, the blue script is not the tilting scribble Doncic used during his teenage years.

Although player autographs evolve and handwriting analysis is subjective, the conjecture has become a powder keg for the sports card industry, which has thrived during the coronavirus pandemic.

When live sports went silent last year, some people discovered the drama of watching others rip open expensive packs of cards on YouTube. Speculators stripped Target and Walmart shelves of lucrative boxes, flipping them for fivefold prices. Bolstered by stay-at-home orders and stimulus funds, the frenzy shared the get-rich-quick impulses that propelled cryptocurrency and meme stocks.

Investors also flocked to grading companies, which by authenticating autographs and rating cards as pristine — sharp corners, smooth edges, perfect centering and an unmarred surface — can spin cardboard into gold. The demand was so great that Professional Sports Authenticator, which charges at least $150 to grade a card, temporarily refused most submissions.

Few of the companies that benefited as money poured in to the collection industry are willing to discuss the ecosystem of athlete autographs, which are used as a key way to tantalize customers. Among collectors, autographed cards found within sealed packs are called “hits.”

Rumors of ghost signers spring every so often, with the signatures of workaday players and superstar athletes like Shaquille O’Neal and Cam Newton sometimes questioned. This summer, collectors were startled by apparent similarities between the autographs of the Charlotte Hornets teammates LaMelo Ball and Miles Bridges. And the companies that make sports cards — and imprint them with a guarantee of authenticity — have acknowledged a few cases when athletes did not sign their own cards.

“This whole thing is just an honor system,” Adam Gellman, who runs the blog Sports Cards Uncensored, said of how card companies like Panini obtain most of their autographs through the mail. “Historically, players have abused it to the nth degree.”

Early doubts regarding Doncic’s signature were highlighted in an extensive Blowout Cards forum thread in early 2019, before the promising Dallas Mavericks rookie had generated the stream of triple-doubles that amplified fervor among collectors. The user who started the thread had previously identified fake basketball cards from the 1990s that were of such high quality that they had fooled grading companies.

Card aficionados traced the evolution of Doncic’s autograph, debating whether the “Luka” signature he has used in person — which slants to the right, with narrow letters and significant peaks and valleys — could be from the same hand as the symmetrical, loopy cursive known as the “Lulu” signature. (There is a wide spectrum of universally accepted Doncic autographs, and not all “Lulu” signatures have drawn suspicion, but the questioned ones are in that script.)

It is practically impossible to prove who signed a particular card; without video proof, not even a “Sasquatch” signature could be unequivocally discredited. Yet that has not stopped some collectors from speculating that Doncic’s mother is responsible for the “Lulu” signatures, which they describe as more feminine.

There is zero evidence for that specific theory, which has become a pervasive inside joke in the industry, but the larger skepticism surrounding the “Lulu” autographs has persuaded some people to purge those versions of the cards from their collections.

Doncic declined to comment, a Mavericks spokeswoman said.

His mother, Mirjam Poterbin, said the idea that she had signed any of his cards was a crazy rumor. “I don’t even know how people can say things like this,” she said, adding, “He’s probably changing his writing — I don’t know. I don’t know.”

Many star athletes, including Michael Jordan and Patrick Mahomes, have simplified their autographs, with the different signatures containing underlying consistencies, such as the relative heights and widths of letters. Skeptics of the “Lulu” signatures argue that it is unusual for an autograph to become neater and to take longer to write.

Three forensic handwriting analysts with no ties to the sports industry disagreed when shown examples of the “Luka” and “Lulu” autographs. One said no determination about their authenticity could be made. One said they were unlikely to be written by the same person. And one said the signatures were generally consistent. A common thread, each expert said, was that the simple four-letter autograph would be easy to forge.

The “Lulu” signatures are primarily on cards printed by Panini, which holds an exclusive license with the N.B.A. and directly reaches contracts with athletes for their autographs; it announced an exclusive deal with Doncic this year. Panini referred questions to a public relations agency, which did not answer inquiries about the authenticity of Doncic’s signatures, or how the company validates autographs.

Several months after the buzzy forum thread in early 2019, Upper Deck, a Panini competitor, posted an Instagram video of Doncic signing cards in its Exquisite Collection. For several interested observers, the swift strokes that produced two long, angled consonants in “Luka” also sharpened Occam’s razor.

“When we hear of issues where authenticity is being questioned, we like to do everything to let people know they’re getting the right thing,” said Chris Carlin, Upper Deck’s head of customer experience.

Top rookies often sign cards at in-person promotional events. But it is otherwise common for athletes to privately sign sheets of stickers that will be affixed to cards, along with a legally binding affidavit that promises the autographs are theirs.

When Upper Deck receives a stack of signed stickers through the mail, Carlin said, the company goes “through it with a fine-toothed comb,” rejecting those that have smeared in transit or that raise authenticity questions. “Usually we eliminate it before it ever gets out to the market,” he said.

Yet there have been times when companies have recalled autographed cards. In 2017, Panini said that in “an extremely unfortunate situation,” N.F.L. defensive end Takkarist McKinley was not always the person who had signed his rookie cards “Takk.” Two months later, Panini recalled some cards of Cowboys quarterback Dak Prescott, announcing that it chose to remanufacture them “after being contacted by an autograph authenticator and following an internal quality control process.”

Now the rumors about Doncic’s autographs have some collectors wary.

When his official memorabilia website offered signed photos as a promotion last year, a Facebook thread was inundated with questions about their authenticity. The store, which is based in Slovenia and did not respond to requests for comment, dutifully responded that it had personally witnessed his autograph session.

“Luka has really special way of signing,” it said in one reply. “If you compare Luka’s signature a year ago and today, yeah it’s different. It’s just the way he does it. Even his signature today is different then 8 months ago. Who knows with what he will surprise us in future.”

Doncic was named the most valuable player of the Euroleague at age 19 and had immediate success in the N.B.A., making two All-Star teams after being named rookie of the year. This summer he led Slovenia to a fourth-place finish in its first Olympic basketball appearance.

“He cares about one thing and one thing alone, and that’s winning,” said Doncic’s agent, Bill Duffy, who added that the athlete did not relish the growing off-court obligations.

“Quite frankly,” he added, “everything else is just burdensome.”

Asked directly whether someone other than Doncic was responsible for any of his autographs, Duffy said the accusation was “false” before deferring to a spokeswoman for the agency, who said, “There has been no fraud, whatever the word is, with any of these signings.”

Collectors who agree that the “Lulu” signatures are legitimate point to a gift for the Slovenian president that features one, a skipped pen stroke that has been observed in both archetypes of autographs, and the fact that grading companies authenticate them.

A spokeswoman for Beckett Grading Services said in an email that outside speculation about Doncic signatures did not influence its autograph experts, who consider “the letter shape and formation, the pen pressure, the flow, rhythm, conviction and spontaneity of the signature, and letter size and spacing to determine if it is consistent with known exemplars.” Professional Sports Authenticator declined to comment.

Another common defense of the “Lulu” signatures is that the variations can be attributed to fatigue from frequent signings. Industry experts said that it took about an hour to sign 400 stickers and that Doncic might have signed at least 10,000 as a rookie.

Gellman, who runs Sports Cards Uncensored, dismissed that explanation, noting that he once watched the quarterback Johnny Manziel replicate his intricate signature for four hours.

“Athletes are required to sit and do this for so many parts of their life that it becomes secondhand to sign everything the same,” Gellman said.

Ultimately, the rumblings about Doncic’s signature have not dulled the top end of his card market.

Nick Fiorella, an entrepreneur who puts most of his disposable income into sports cards, said his riskiest purchase was the $4.6 million Doncic card with an N.B.A. logo and a “Lulu” autograph. But he is betting that the player and the hobby will continue to soar.

“To me, if it’s him or his mom or whatever, it’s always going to be his one-of-one,” Fiorella said. “If he becomes a transcendent player, it doesn’t really matter if I signed it.”

Sheelagh McNeill contributed research.