Moisture problem or not?

Hello everyone,

a question from a construction novice... It's about the ceiling in the bathroom.

First of all, the general structure:
Single-family house, shed roof with external insulation and exposed roof truss, exterior walls made of aerated concrete 40cm (PP2, lambda 0.09), interior walls made of 11.5/20 sand-lime brick, no additional insulation on the exterior walls, underfloor heating, no controlled residential ventilation. Building application has been submitted.
The exposed boarding of the shed roof extends beyond the wall crown. The planner's design provides for the transition from the wall crown to the rafter and exposed boarding to be sealed airtight, but with no additional thermal insulation. In my layperson’s opinion, the continuous wood of the rafters and exposed boarding is a thermal bridge where condensation can occur. Unfortunately, I have no scientific arguments to substantiate this. And I have no better idea to eliminate this weak point.
My main concern is that the bathroom on the upper floor might have a permanently cold spot in winter at the transition between the wall and the exposed boarding. This could lead to a slight problem with mold in this damp and cold spot during normal bathroom use with 4 people in the household. At least I fear that and have informed the planner accordingly. He is now trying to portray this concern as exaggerated. My request for a better solution has not been addressed.
Could one of the experts tell me if this is a problem or if I am worrying unnecessarily? Or can such a transition from exposed boarding to wall be done differently/better?
I am grateful for any tips!
AT

is it sensible to additionally install an insulation beneath the rafters in the bathroom besides the roof insulation? that way the visible boarding boards would be behind an insulation again, and the problem with the thermal bridge and condensation would be gone. or would that cause other problems?