Hello and welcome to the forums.
If the heater control valve for the heater core is stuck in the closed position the coolant might stay cool at both sides due to the coolant not being able to flow.
There was a member on here who realized theirs was stuck closed when they bypassed that valve with a piece of hose and got heat back. I'm not sure how mechanically inclined you are and I'm not saying it's going to fix the problem but it's a good test.
If you do flush the coolant, which VW says should be done every 30K miles or 2 years, it should be refilled using a vacuum type of tool so as to not get any trapped air.
If the heater control valve for the heater core is stuck in the closed position the coolant might stay cool at both sides due to the coolant not being able to flow.
There was a member on here who realized theirs was stuck closed when they bypassed that valve with a piece of hose and got heat back. I'm not sure how mechanically inclined you are and I'm not saying it's going to fix the problem but it's a good test.
If you do flush the coolant, which VW says should be done every 30K miles or 2 years, it should be refilled using a vacuum type of tool so as to not get any trapped air.