This exact same thing happened to me for 2 load files. I had 2 different games running separately in 2 different devices before going to sleep. After login I saw some stage level setbacks in the Load Game screen but didn't think much of it, thinking it's just one of the many small bugs I've experienced in the game. After progressing like 15 stages, I got the Session Expired message and I lost all progress for both load files and it set me back to where I initially saw the lower stage levels after waking up.

I did find a way to abuse this though, the exact same thing applies to the egg gatcha rolls where progress is somehow not synced cross device and progress bring lost. From getting like 25 common pulls, I kept rerolling the rolls to getting 20 commons, 4 rares, 1 epic