Auto-lock problem (iOS)

I performed a research but not really a lot of bugs are found. Also, I am not very sure if this is intended

The Auto-lock feature on my iPad is set on "2 min", meaning that the device should be locked after 2 minutes of no activity. However, on the world selection screen and home screen of the game, the feature does not work: sometimes I discover that my device is still lit after I come back home in the afternoon after I played it in the morning. However, if I open a world and plays in it, the device would be locked after 2 minutes.