Are you sure you're not hitting the Reset Text Size option? I don't have that exact Android version installed on any devices currently, but I haven't heard of any problems with this.
The behaviour is consistent every time, and the text jumps back to the previous size at the same time as the sub-menu opens. I.e. before even trying to select save or reset.
That is how this function works -- you need the Monospace font for accurate alignment between lines.
Ok. I'm coming from other chordpro-apps such as songbook for iOS/OSX/win/android where chords and lyrics are aligned regardless of font.
Starting from the main menu, what buttons would I have to press to see the problem?
Repertoir->Setlists->choose any>
With the setlist displayed as "list only", press the document symbol on any song in the list to display the lyrics. On the fist attempt the lyrics page is briefly displayed, then the app drops back to the playlist.
After this you can go back and forth between the playlist and lyrics with no issues, until you close and reopen the setlist at which point it happens again, once.