I've seen similar behavior on private servers, and in my case it was a result of mapView in redis being outdated, non-active rooms are not processed near as often as active rooms, so their mapView entry doesn't get refreshed. A simlar issue occurs with portals if the room isn't toggled active after spawning, portals can also end up with negative decay if the room hasn't been active since the time elapsed. (Creeps and owned structures make a room active, but it goes inactive automatically when none are in it.)