When an activity has more than 1 teacher and these teachers have their own homerooms when the generation button was clicked, no rooms were selected and it will be an available time slot for other activity to use it.
Actually, it is not a bug, but a feature:
1) If you want an activity to occupy two rooms or more, you need to use virtual rooms or separate the activities and add a constraint same starting time.
2) An activity with two teachers or more will not respect home rooms. I wrote that somewhere in the help. You need to add preferred rooms constraints for those activities (using preferred rooms for activity, for subject, for subject+tag, or for tag).