I don’t think this is a problem. The drop is only by 0.5° and the return is dropping by the same amount. This just means that the heat pump is trying to match the heat loss, but hasn’t quite managed to lock it in perfectly, so output is a bit less. Around 22:00 it notices, and ups the heat output a little to compensate. COP is a steady 4.0 between changes in input.
This looks to be working as it should, within the capabilities of the controller. If you want the flow to be exactly 36°C, you may need to increase the target by a degree.
It’s not looking great, no, but your experiment from cold showed that good performance was possible.