HI. I’m using the default and have never turned on FSRS.
I updated my Anki and also installed an add-on quite recently. And the time when the problem started roughly matches with the time of this update and installation.
But I have deleted the add-on and restarted my computer and everything. So I believe the add-on had nothing to do with this problem.
What do you have set in your Deck Options for the “Easy Interval” [probably best to just screenshot all of the intervals/modifiers, including the Advanced ones]? Are all of your decks using the same Options preset?