I also suspect a sensor defect.
You wouldn’t believe what kind of (partly) unexplained phenomena can often occur with simple sensors ;)
Especially with a heat pump.
With cheap sensors, moisture can form inside the sleeve, which affects the resistance/value. This often happens during the transition period (cold at night, warm during the day).
The constant, slight vibrations of the heat pump can also cause the connection between the sensing element and the cable to come loose.
It has all happened before...
That’s what you get when you squeeze every cent out of the supplier ;)