It’s not really broken. Your party members are completely separate and distinct characters that can do whatever they want from a role playing perspective, including splitting up - arbitrarily adding everyone into initiative even if the enemies can’t see the other PC’s would feel less realistic and more videogamey. I’m saying this as someone who plays tabletop dnd and values the flexibility and near limitless options the game can accommodate. This is also a weird situation cuz many tabletop players have very different homebrew rules about how combat is initiated. Some DM’s allow people to opt into combat when someone else initiates, some DM’s decide who is in an encounter by distance, some force everyone into initiative when one party member initiates - there’s a lot of permutations and I think Larian did the right thing here cuz if you don’t like it and think it’s broken you can very easily not use stealth and line of sight to nuke your enemies. This isn’t a competitive game after all. I for one appreciate the tabletop spirit carrying over to BG3 instead of being unnecessarily restrictive.