I’m not seeing what the problem is.
When you first start the car, the coolant is cold (ambient air temp), and the engine will take a while to heat up the coolant to the point where you will be able to feel it.
Now, if you are saying that after you get off the highway when the heat was working fine, and now are stopped at the first light, and the heat slowly vanishes, then yes, you have air in your cooling system, and need to do a specific air bleeding procedure to get all of it out.
BC.