I do think there’s another bug to be found. Here, I’ve always wondered about the fact that number of items to be reviewed differs between my tablet (which I use most of the time) and my phone (which I rarley use, mainly when I’m waiting somewhere or commuting).
I’ve never really compared both devices at the same time, but I’ve seen more than hundred words in a course’s backlog on my mobile when I had reviewed everything (i. e. 0 remaining) an hour or so earlier on my tablet (Android on both devices).
Recently there have been several discussions about grammar mode, which exists in new courses. I answered a post from a user who studies the same course as you. I copied below. At the end of the text, there is a link to the Joshua post (Memrise team), which helped to clarify the bug.
