The mouse sensitivity resets when logging on to a world in 1.17. Noticed it instantly because I play with a higher sensitivity. Doesn't show it being reset in the mouse options menu but is definitely slower than normal. It is fixed by just setting the sensitivity to the preferred level in settings again. It happens every time I relog onto a singleplayer world. 

Edit: It isn't when opening worlds, it is kind of random but happens often during gameplay. Don't know if it happens for others though. 

Never mind it is my mouse, I don't know how to delete a post