Why does my game lose progress when idle in background?

[AC Question]

Game loose progress when idle in background

Game closes when idle in background.
Android 5.0.2
Samsung Galaxy S6

If I am in the middle of playing a Level and I go away from the game by pressing the main phone button or opening another app and then when I go back to the game after a while, the level & the app restarts - as in I see the Game logo and have to click on Play Level again - I miss my progress for that level.

Any ideas please?

Thanks,
KB