Increased supply temperature in underfloor heating during ventilation

The screed stores significantly more energy than your room air..
By the way, also the reason why heat recovery in ventilation is rarely economical.

However, the issue is less about constant storage and more about the dynamics, i.e. the time derivative of these quantities.

And indeed, the screed probably loses less than an air column inside the house, which is in an open relationship with the outside world

Since the ventilation system primarily serves a comfort purpose, the additional cost for actual heat recovery is almost negligible. And even saving about 1/4 of the heat demand in Kfw 70 (typically 100 to 150 euros) accumulates over the years.

I would appreciate clarification!

I know the phenomenon from our old house and it was indeed due to the room temperature sensors being exposed quite quickly to fresh and cold air during ventilation, as already suspected. You can try the following experiment: Insulate the temperature sensors from the room's air volume with a bag or can while you ventilate. If the phenomenon does not occur again under this arrangement, you have found the reason: sensors that react quickly coupled in a non-ideal way to a sluggish system. In our house, the effect was annoying because rooms could overheat with a delay following ventilation - however, this underfloor heating was from 2001 and certainly nowhere near the level of today's systems.