Note that you may not be able to break a switch to get the zombies to spawn in if you are on a save from before that fix was put into the game (I believe it was added in the last experimental version before stable).
They are aware of this issue and are trying to resolve the problem. If you are willing, add your own comments to the bug with details of what happened so they might have more info to help find the cause of the bug. I think Jugginator or someone asked for more details in a forum post but not sure where that is. Adding details to the comments of the bug should be fine, though. Anything that might help - did you complete the quest before? Did you loot the POI before? Was there any unusual amount of destruction of the POI, such as blowing holes through the walls with a shotgun or breaking doors that may have contributed to the issue? Any other details would be useful for them, I think.
EDIT: I found the post. You can also just add your details to this thread. The link points to where Jugginator asked specific details about when this bug happens.