Some people have this problem, their save somehow become corrupted, they lost some or all characters and progress. I have a new phone and i lose the save frequently, i didn't even have to play, it is enough if i exit on the title screen. Even the game settings (volume, speed, etc) will be reseted by this problem to the default. I think it is happening because i left the game in a not intended way or maybe the game is killed by the system before it could save properly.
When i left the game, i just swipe out it from the recent apps overview screen. I read about it, swiping kills the process, so i would be not surprised if this could corrupt the save.
Also i tried, the home button just sent the game to the background without saving. If i reopened the game, it continued from where left it. Swiping the game out after i left it with the home button can corrupt the save too.
I tried few dozen times, if i left the game with the android back button to the android home screen, the game saved properly and i can swipe it out without problem. Even if i don't swipe it out, the next time the game starts from the title screen, i see this as an evidence the game properly exited when i left it. This is the "workaround", back button is a safe way to exit the game.
I know nothing about the android programming, but out of curiosity i read about the swiping, hopefully i'm not totally wrong. It seems swiping kills the process, but the document about it is vague, some says it is even misleading, the calls and methods changed over time and many developers are unsure how they should catch the swiping event, because there is no (guaranteed) call when this happening. Some developers suggest the onPause method because it is not killable by the system and called when the user press the recent app overview button before swiping. This
android developer document support this, as onPause is a safe method to write persistent data to storage.