Setting the heater to “heat” position, which blows to foot and when pressing gas pedal to “kick down” / hard acceleration, the heat start to come from upper vents. Heater speed is also increasing. It looks like the system loose vacuum somewhere, but cannot identify. Checked for vacuum leak at engine compartment with smoke - no leak identified.
'Supposed' to is the key word. I've seen a few controls where the seals in them or the vacuum lines to the various diaphrams no longer have perfect seals. Then they lose vacuum even with a perfect check valve.