I think this is just a major bug with stealth and sight. I have this happen a lot too. Astarion will succeed a save only to be forced to constantly reroll until revealed. I don't know if its a known issue or intentional.
That's what happens when you're not in turn-based mode.
Is it possible that some of your characters went in turn based mode, but others didn't because they were outside of the chain and you didn't realize it at the time?