It seems that BG3 follows the same system as DOS2 in that stealthed/hidden characters are not in initiative order once combat commences. This results in a clunky situation where the hidden character is in their own little world whilst everyone else is taking turns around them. Even worse, if you get spotted whilst trying to act the game just slots the "new" combatant at the end of the initiative order which can be disastrous.

An example I've had of this is in the Chapel entrance fight, Astarion was able to sneak around the entire group to backstab the caster during Shadowhearts turn. In my opinion this just breaks the whole point of having an initiative order and can lead to situations where combats are cheesed by exploiting being hidden.

I believe all combatants should roll for initiative, hidden or not and then act as normal on their turn in regards to action economy to avoid these janky scenarios where a hidden character is doing whatever they want while the rest of the world is paused around them.