I believe this is caused by the orange pile not updating when a card is scheduled for review while you’re still reviewing cards. Thus, when I’m at zero cards and I get a card right, anki decrements that pile assuming the value is a correct and up-to-date count of how many cards are left to review.
This error only occurs after I answer a card and the orange pile was at 0. It doesn’t matter if I mark my answer as correct or incorrect. If I press “sync” that will reset the orange pile to the correct count.
I suspect it may be due to my settings. In particular this page:
I also probably have some unusual deck options, which I will post for the sake of completeness even though I doubt its related (some images removed due to max 5 embeds per post):
Version 2.1.47 (dc156f34)
Python 3.8.1 Qt 5.15.1 PyQt 5.15.1