@only Hi!
There is some news to that “Fill”-button, see here: Usability issues (UX) in redesigned UI (2024.12.0) - #550 by Nail1684
PS: Also check other auto-fill methods as an alternative: PSA: How to easily autofill using the new browser extension UI
There is a more comprehensive list of some settings, if you want to get the UI/UX closer to the old one: Usability issues (UX) in redesigned UI (2024.12.0) - #107 by FaviFake