Underfloor heating and heat pump. I thought I understood it

Do you have many windows where the sun shines on the floor?

Throttling everything is not ideal. Better to reduce the pump strength. Or is that not possible with the heat pump?
Which one is it anyway?
How big is the house?
Is an overflow valve installed?

@TE: Please describe your system first. Only mention the relevant things.

Stiebel Eltron LWZ CS5 Premium
150 sqm, KfW55, air-water heat pump with central ventilation.
2 heating circuits separated by floors.
No overflow valves.

Rather: did a heating cycle run at all?
The 0.2K deviation is simply the inaccuracy of the sensors, or also simply due to the installation location.

Today the same behavior, a heating cycle definitely ran.

Do you have many windows where the sun shines on the floor?

Relatively few. Especially since there was little sun in the last few days.

If I may interject here with a question (my planned house is supposed to be very similar to that of the topic creator: 140 m², air-water heat pump + centralized ventilation): My house construction provider says that for KfW55 with a basement, underfloor heating is required everywhere if I use at least one basement room as an office/workroom. He said this has to do with thermal bridges which then cause problems in the calculations if the basement is cold. Could that be true? And besides, I only have one heating circuit planned for all floors (UG = basement, EG, OG) and a flow temperature of 35°C (!). All of this seems strange to me...

Certainly, one heating circuit distributor is planned per floor and they are connected in one heating circuit. At least that's how it is for me and it works very well. 35°C flow temperature is not very ambitious, but do you have influence on the planning? If the basement is planned within the thermal envelope, it must be planned with heating.

No pressure relief valves

Sure? Then you have a buffer tank, right?

Same behavior today, there was definitely a heating cycle running.

So the compressor was running? Then that actually can't be. Please take a photo of the display in the middle of the heating cycle, maybe also of the radiator control valves.

Stiebel Eltron LWZ CS5 Premium 150sqm, KfW55, air-to-water heat pump with central ventilation. 2 heating circuits separated by floors. No bypass valves.

And where is the big rest? „2x the half“ is still missing.