I know it's celebrity news, so might not be true but the story of his special contract clause has been around for a while now.

"During our filming, JM closed the Kang deal. I remember asking said friend about it, and they shared some details. X number of movies, X number of dollars, and more interestingly, due to the nature of the character and potential multiple versions, JM had a contract clause stating that only he could play any and all versions of Kang.”

https://www.sportskeeda.com/pop-culture/marvel-forced-keep-jonathan-majors-kang-the-conqueror-leaked-contract-shows