This could be a few different things. First, if the engine is reaching operating temperature and the engine coolant is full, the first suspect based on your description is the heater core is partially plugged.
On the other hand, the air temperature in the vehicle is controlled by what is called a blend air door. There are actuators that move the doors to allow different amounts of heat or A/C to enter the vehicle.
If you can, try something for me. Drive the vehicle with the heater on. When the heat is working normally, open the hood and feel both heater core hoses. Both should be hot.
Close the hood and keep driving. When it cools off, stop, open the hood, and feel the hoses again. Is one a lot cooler than the other?
Let me know what you find. Also, here is a link you may find of interest:
Also, let me know if the vehicle has automatic climate control and dual climate control.
Take care and Happy Thanksgiving.
Wednesday, November 24th, 2021 AT 10:28 PM