Greasy Fork is available in English.
Removes the "Important" section in the YouTube notification menu, putting the notifications whithin it back to their appropriate position in the notification list.
< Feedback on Remove Important YouTube Notifications
I also experienced the issue of disappearing notifications today. I logged some variables, and I hope this helps you identify the problem.
After these logs were printed, similar content stopped appearing, so I suspect that for some reason the MutationObserver stopped being triggered.
Currently running well on Firefox, truly grateful.