Looks like a problem with the flow meter, possibly trapped air, as the flow temperature continues to increase. Not immersion. Heat pump is fine, problem is with the measuring.
Just need to keep running the system and bleeding the radiators and auto air vents.
Related: Kamstrup Multical 403 Stops Working During Hot Water Cycle (Interference?)
And: Kamstrup Multical 403 Erratic flow readings