According to a video posted to social media on Tuesday, a fan at one of the club's official outlets asked an employee why he was unable to purchase a Lewandowski shirt.
"We don't have any more of the letter W, and Lewandowski has two. It's sold out," the staff member replied.
A Barcelona spokesperson confirmed to ESPN that the issue had arisen, and said the club expected it to be resolved in all stores by Thursday.
The departure of Senegal international defender Moussa Wague this week left forward Martin Braithwaite as the only other player in the Barcelona men's or women's first-team squads whose name includes a "W."
The letter W is extremely rare in Spanish and in Catalan, with the Royal Spanish Academy (RAE) saying the letter's use is restricted to "words of Germanic origin such as Wagnerian... or Anglicisms, as in Washingtonian or whiskey."
Barcelona have defied their tight finances to pull off a string of deals this summer, with teenager Pablo Torre, AC Milan midfielder Franck Kessie, Chelsea defender Andreas Christensen and Leeds winger Raphinha all arriving.
None of those signings, however, has generated as much enthusiasm as Lewandowski's, which is understandable given that the 33-year-old is one of football's greatest-ever goal scorers.
The Poland international was runner-up to Lionel Messi for the 2021 Ballon d'Or, won the FIFA Best Men's Player award in 2020 and 2021, and took the Golden Boot for Europe's most prolific finisher in the past two seasons.
His arrival was made official on Tuesday -- after a deal in principle was announced at the weekend -- with Barca paying Bayern Munich a transfer fee of €45 million, plus a potential €5m in variables, and the player signing a four-year contract with a €500m release clause.
Lewandowski flew to Miami, where Barcelona have been based for the start of their preseason U.S. tour, to meet his new teammates and pass a medical as the final details of his contract were agreed.