Exhaust fan dripping?
In my state bathroom exhaust fans are required to stay on 15 minutes after the lights go off for this reason.
Actually the time allows the excess humidity in the bath to be removed and the dry air blowing over the fan and duct help to dry it out also.
We don’t hear about it often on this site but prior to the 15 minute requirement new homes we built did have some of the same issues.
Increasing the home ventilation or make up air and the 15 minute times really has eliminated this issue I haven’t received a call back for this in years.
Add a fan timer to your switch and it will probably eliminate the issue since it sounds like you have good air flow.