So this is a really old bug what is in the game again after TU15.
Basically after a short period of time the game starts to drop the FPS value from 60 all the way to 20 Fps and so can't keep the 60 Fps anymore.
60 fps and then 50, 40, 30, 20.
The problem is the Ubisoft Connect app. The FPS drop issue can be solved, if you open the Connect app on the main pause menu in the game and then properly close it,
after that everthing goes back to normal, 60 FPS again, but come on Ubi this is just ridiculous tbh.
So please Ubisoft do something with that Ubisoft Connect thing because this is just so annoying, i mean an old BUG after 3 years is here again!?