There is no historical evidence or documented research suggesting that Juan Ximénez de Tapia was dyslexic. However, some collectors and researchers have speculated that the irregular and reversed placement of letters and numbers on coins bearing his assayer mark ("T") might indicate difficulties with writing or engraving standards. 
That said, such inconsistencies were fairly common in colonial mints due to handcrafted minting processes, illiteracy among workers, and the rushed production of coinage rather than any confirmed neurological condition.
Assayer Tapia was known for his work at the Potosí Mint in the 17th century, where he was responsible for overseeing the quality and production of silver coinage. He played a key role in the infamous Potosí Mint scandal (circa 1649), one of the largest corruption cases in colonial Spanish America. During this time, mint officials—including assayers—were accused of debasing silver coins by reducing their precious metal content while maintaining their face value. This led to a major crisis, with counterfeit or underweight coins circulating throughout the Spanish Empire, ultimately prompting a royal investigation and severe punishments for those involved.