I also had the foam desintegration issue, and what it does is that there is no proper air sealing when the flaps are closed. Meaning that you will close air to specific vents, and there will always be a small amount of air being blown. IMO, that won't, however, affect temp changes.
I would check for a clogged heater matrix. Just flushing the coolant system might not be enough (been there, done that). Overtime, dirt accumulates inside the heater matrix narrow passages, preventing water from circulating. Despite the hoses getting hot outside, water won't flow. If this is the case, unplug the hoses and try using compressed air (carefully), in the opposite direction of water flow; also be careful with the valve described in the paragraph below, that needs to be open.
There is another possibility. The temperature is regulated by a vacuum operated thermostatic valve (controlled by the temp setting), located in the motor compartment, in the heater matrix exit hose. I believe this type of heat control started being implemented during year 2000, so older models might not have it. If the air operated diafragm has ruptured, or there is a vacuum leak, there will be no temperature changes, as the valve won't operate. You can easily test the valve applying vacuum, as you can listen to it open and close.
In either case, however, and despite water flow is blocked (by a clogged heater matrix or a non functioning valve) you could perhaps feel a mild temperature change in the air leaving the vents, as heat propagates through conduction to the heater matrix. One easily assumes that there are no temperature changes, as it takes a very long time for heat to reach the heater matrix, and any effect, however minor, to be felt.