Android: 5.1.1
Baseband: G900TUVS1FOL1
Kernel: 3.4.0-5406928
Device: Samsung Galaxy S5 SM-G900T
It happens to me quite often, although I am not able to reproduce it via a pattern yet. Basically, when I am creating a mem, I usually switch to my Android Chrome browser or Google Translate app for research. After certain time (ranging from couple mins to half hour or so), I switch back to Memrise and think that I should be still at the mem creation screen. But sometimes it just bring me back to the Memrise home screen. Yet some other times Memrise simply has quit itself. 
I doubt if it’s due to session timing out as sometimes it just takes a min or 2 to behave like this.