It seems like most of the series that I mark as "Unfinished" instead get marked as "Complete." User error would account for 1 instance, not this many 😔

I'm sorry to hear that, I'll take a look and see if I can replicate it. Could you give me the steps you used to mark it as "Unfinished"? Are you doing this from the list page, series page, or somewhere else?

Developer for MangaUpdates (Web, iOS, Android)

Looks like I wasn't subscribed to this topic, and never saw your reply. I just replicated it again. The issue can be tested in either the list or the series page.