I am experiencing the exact same issue.
When I am on the UFC 6 main menu or in the Online Career mode lobby, my PS5 Slim fan spains at an incredibly high speed with massive noise. Shortly after, an official system warning pops up stating that the console's temperature is too high.
The ridiculous part is that once a match starts and i am actually playing the game, the fan noise slightly decreases and stabilizes compared to when i am just sitting in the lobby.
My PS5 Slim is perfectly clean and free of dust. Furthermore, when I play other graphically demanding, high-end games, my console remains completely stable and the fan noise barely audible. This is clearly a optimization issue with the UFC 6 game itself.
EA needs to fix this problem as soon as possible. I am going to stop playing UFC 6 until a proper patch or action is taken regarding this issue. Otherwise, I genuinely fell like this game is going to break my console.