It could also be that the heating loops of the other rooms run through the bedroom? Where is the HKV located? Unfortunately, that is sometimes done incorrectly as well. That would be extremely bad m
One could possibly also reduce the charging pump at night (level 1 or if stepless then about 50%) so that less flow enters the upper floor. Of course, this has to be adjustable on the control system. The disadvantage would be that overall less "pressure" is built up on the entire upper floor and accordingly less heat comes in (e.g. into the bathroom).
Alternatively, it would also be possible to install an additional drywall in the bedroom on the interior walls. On the one hand, this can compensate for heat loss (to the adjacent rooms) and if you pay attention also insulate against noise (if that's needed;) )
It is simply the case that the temperatures inside the house equalize. The houses are well insulated, so the surrounding rooms help heat the bedroom. Otherwise, you would eventually have the AT in the bedroom. I am probably different here than most. Even in summer, I need something cozy, and at 20 degrees in the bedroom, I feel totally comfortable with a normal blanket. It could actually be even thicker. I mean the blanket. Not me :)