I kinda did answer your question though? In my experience, I've had plenty of runs where a guy yolo pulls the instant he's close enough, and most of the alliances aren't in the room. I've literally had groups wipe because so few people were in the room that the fight insta ended itself. Point is that, in more than one of my runs of that raid, most of the group wasn't in the room when the guy got pulled - and A tends to end up with at least 3-4 extra people in it just about every time I run it.
I'm not just talking about myself here, lmao.
^Also this. Some people load slower, some people were further away from the exit when the last fight ended. It happens. Often enough to be noticeable.
Charging at the boss (or Atomos) when half the group isn't in the room helps absolutely no one. Wait a few seconds. It isn't gonna kill anybody - but rushing just might.