Many cars use vacuum to move the blend doors to determine where the air flows.

The vacuum comes from the engine, but only at certain throttle position/RPM (when the throttle is open, little or no vacuum is present).

So cars will have a small plastic vacuum "ball" hidden somewhere inside, with a checkvalve. That ball will provide enough vacuum to keep the blend doors working.

But it's common for the ball to crack, or rub against something and develop a hole. Or check-valve can also fail. In all these cases, the doors will only move while throttle is closed, usually with higher rpm as well (so a quick blip of the throttle to get rpm up, and release throttle quick, resulting in higher RPM with throttle closed)

For ChickNorris' issue, since it happened immediately after replacing a heater core, I'd bet a vac line or electrical connector got disconnected or knocked loose.