Yes, the game handles this sort of scenario very poorly.
See, eg,
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=839959&page=1 for more discussion of problems related to combat not triggering for all party members. These problems do take a number of forms, and it’s true that the game doesn’t always force a stealth check.
If you try to play the game being as stealthy as possible, it’s guaranteed you’re going to come across broken, dodgy behaviour. It’s why I don’t do it, though I wish I could.
