This has happened to me a few times in the last week, but I finally paid attention to it today so I could record exactly what happened.
I had 205 cards “due” at the start of the day. I immediately reviewed all of them, and got 20 wrong. When I go back to the main screen, the “Due cards” button initially shows 20. However, after about 30 seconds (the “two arrows” icon in the top right of the app was spinning), that number changes to 115! If I try reviewing again, many of those cards are ones that I just marked as “got it” or “easy”. I don’t know what will happen to these cards tomorrow – if I have to review them again or they magically disappear. Either way, it’s quite frustrating.
This is version 3.4.1 (300389) of the beta app on Android 9. Happy to provide more info if it will help.