Hi, I know this thread is old, but I've encountered this issue too multiple times in the past (since 2015, I think?) and apparently the problem still persists in 2021.
I'm using version 7.49 on a Galaxy S20 FE with an Android 11. Just now I recorded multiple acappella tracks and was editing it. I minimized mixpad into the background for a bit (I've done this multiple times before this point) to pair my bluetooth earphones. When I opened it back up, all my audios flatlined, the undo/redo buttons are inactive, and the history only has a single "Open Project". The tracks placeholders are still in place, the names are still correct, and the effects are all still there.
I know I can't salvage my audios and need to redo everything from scratch. I'm just here to let developers know this problem is still happening years later. I hope it can be fixed but for now I'm uninstalling.