Lost a couple of arena games in a row this evening, and 3 yesterday because of this.
Mid game if a message is sent to me (eg Facebook messenger) the notification pops up at the top of the phone screen. This causes hearthstone to boot me to the main menu. It doesn’t seem to recognise it’s in mid game. The only way to get back in is restart the app but by then it’s too late to continue the game as I’ve lost 2 turns.
Seems any push notification will do it, not just fb.