However, they do need a legitimate path in order to focus. So if there's an invisible character blocking the doorway, then the monsters can't focus on that character (duh!), and can't path through them to a space from which they can attack characters in the other room, so don't focus on them either. Which then means they don't move.
Note that archers might have sufficient range to be able to shoot past the invisible obstruction. If so, then they can find focus, and move.
Also, Flying creatures would find focus and move without issue.