I don't know if there has been some changes to Renshuu as of late, but just now as I logged in there were a bunch of new unstudied terms on pretty much all of my word schedules.
The only schedule that should have new terms to learn at the moment is N1, all of the others I have cleared some time ago and (apart from the newly made N3 with the grammar companion list which I am redoing at the moment) there were no new terms on the others (basic, beginner, pre-intermediate and pre-advanced) as of yesterday, only review.
Also encountered this issue, although can't tell specifically what exact schedules are affected, since I have merged my N4 and N5, and N3 was still in progress anyway. Based on OP I'd assume all of them, though.
No mastery data has been lost, so no worries about that. I'm rolling in a backup to handle the fixes, and there is no need for manual changes while you wait for it.
I am encountering this bug as well, "Words for Japanese Basics" and "Words for Beginner Japanese" are showing unstudied terms despite me having finished them. So if there was a fix, it didn't seem to work, for me at least.
I seem to have the same issue. When I logged in today, all my schedules, except for one, had no reviews whatsoever which really confused me. When I tried adjusting the pacing on a few of my schedules to fix that, I find myself having the same Issue as OP.
What really confuses me is that some of the "new" terms that show up in the beginner schedules already exist within my learned terms, and in fact have pretty high mastery. But somehow the terms that I've already learned were removed from all my schedules. Example: 9日
"New" term 9日
9日 that I've already studied
Somehow removed from all other lists?
Edit: And apparently a lot of the "new" terms are duplicates as well. The one I've already learned on the right was somehow removed from all my schedules, while the one on the left suddenly appeared in my beginner schedule, even though I've already learned it.
Had to remove a few terms which surely were manually removed the other day from active schedules that suddenly appeared as unstudied although not sure if the frozen schedules are affected
Still seeing the issue on the words for beginner Japanese schedule. A handful of unstudied words plus the 20 or so from yesterday I studied. I think I already had all of these with higher mastery before
