Hi! Thanks for checking out the game. How I've ruled it is if the monster has [stealth] tag, then the mind test to spot them goes first. Then you can make a body test to hide at the beginning of a room (thieves get advantage). If you passed the ambush test then you have advantage on your first attack. If you failed, the monster spots you and you take damage. You can only stealth once per room typically, so a thieves double damage usually only happens one time.
I hope that clears things up a bit!