Because it sounds like your attic is cool and you re in your shower and the bathroom gets super hot and then there s no movement of air in that bath and that s what s causing that temperature differentiation.
Condensation in attic above bathroom.
Insulation is responsible for keeping warm air inside your living space and out of the attic where it can lead to condensation.
To save time and better pinpoint the problem you can hire a professional to inspect your insulation and fill any gaps that might be letting warm air into.
Turn up the thermostat a few degrees in your home to warm the bathroom air to a warmer constant temperature.
The warmer air will help to warm cool surfaces such as mirrors and walls reducing.
Noticeable musky damp odor.
So what happens is the warm moist air strikes the underside of the chillier bathroom ceiling and condenses.
When the cool air from your living space rises due to humidity in your home often from cooking bathing or drying clothes it comes in contact with the warm surface of the ceiling and condensation forms.
It could be a lot of things but the most common source of condensation is a poorly or unvented bathroom.
Inspect your attic insulation.
If you have taken the proper steps to remove excess humid air from your home and you re still having condensation issues it might be time to check the attic insulation.
Moisture forming on the glass window panes or other flat surfaces.
If your bedroom has an attached bath that could be the culprit.
Traces where water has trickled down the walls.