I've not seen it addressed, so I am asking: has anyone tried to fill the gap above the bathroom door? I am trying to think of ways to close it off without harming the walls or ceiling, but can't think...
The mfg put the gap there to make sure there was sufficient room for air flow when you turn the bathroom fan on high. If you block it, there won't be sufficient air flow to suck out odors, shower moisture, heat, etc.