Measurement values natural cooling heat pump

if you don't trust your heating engineer:
Operating instructions
If that doesn't help, call the manufacturer.
If someone here has experience with your heat pump, they can still help you, as the controls are very manufacturer-specific.

The instruction manual is not very informative on the topic, response from the manufacturer is still pending.

The brine circuit is definitely in motion, I just turned on the cooling. The source was at around 20°C, shortly before there was domestic hot water operation and it dropped to 11°C within about a minute. Supply/return stabilized at 24.2°. Outside temperature 21°C inside 23°C.

Strangely, the temperature of the suction line rises from 33°C now to 40°C

That means the brine circuit is running, do you happen to have supply and return temperature sensors there as well, is the heating circuit still running? Have you checked in winter operation if you have a difference between the heating supply and return? What is the room temperature now? Where was the supply/return temperature before switching on? What must not be overlooked: cooling 1 liter of water per second by 1° requires about 4.2 kW cooling capacity (P = V x t x c). No idea what heating capacity you have, but with 8 kW heating capacity and a 5° difference between supply and return, about 1400 l/h come out. That means 0.5° between supply and return would be 0.8 kW or 1° would be 1.6 kW cooling capacity. To me, that means you should always see a little something.

Heating operation runs great, usually around 28°C supply and 25 return. I don't know about the source on/off in winter, with the source temperature there is a difference of at least 0.3°C between inlet and outlet. The source temperature hardly changes in cooling operation.