I find this typical.

Set the dial at 20 and leave it alone. If it's really cold, you may need to push it to 21 or 22..............but massive movements of the dial are contrary to proper function and the system doesn't behave well when you do.

Remember, if the heater core is warm and the system determines that the vehicle needs to be cooler, there is a bit of a delay before all the heat dissipates from the heater core. The system doesn't switch back to "cool" mode if the ambient temperature is cold.

Likewise, if the system determines that more heat is required, it opens the monovalve gradually and the discharge air temperature climbs slightly. It won't blast the heat to quickly bring the temperature up because of the control problems that will be generated by an extreme overshoot.

If you want to heat the cabin quickly with maximum heat, just switch to defrost and raise the cabin temperature by 20 degrees within a matter of a minute or so...........however, the overshoot issue will then come into play.