That's happened to me too. I think it's when you get less than 90% when you're retested on a verse you already finished. e.g. when you review a whole chapter, if you don't remember a verse so well, it goes back to the four-and-half star state, and you have to finish it a second time a few weeks later.
Hmm. That actually sounds very plausible. Just went through a chapter I thought I knew well, and I know it reasonably well but not perfectly. Didn't know you could go backwards, but good to know there's no bug in the system that's going to keep me at 90 forever! Thanks :-)
It is true. It happened to me. I had 5 stars for Ephesians 6:10-18, and declared "finished" and then I blew it at verse 12, I got a 90%, and it vanished from the finished verses.😳