Center dash face vents : Stratified Air Vent control ?

[Stilllearning6]

1998 740iL:

I am trying to understand how the "Red<--> Blue dash dial ", known as the Stratified Air Control dial, controls the modifying of the temperature of the air that blows out of the center dash vents.

Search of the forum, with the keyword : " stratified " , revealed excellent posts by QSilver that informed me that the red-blue dial will modify the master temp setting of only the center dash vents, not the Defrost duct, not the Floor ducts. He posted great air pathway schematics.

My experience was that both Defrost ducts and Floor ducts are blowing super hot air whenever the temperature is set at 90 degrees.
However, the center dash vents still blow only cooler ambient air.
Question: How to find the cause of no hot air out of center dash vents?

I am leaning toward an issue associated with the red-blue dial, not a trapped air pocket issue.
Since I get super hot air at defrost and also at floor, I believe that the system is fully bled, no air trapped.
Is that correct assumption?

If correct, then I believe that the red-blue dial must not be affecting whatever it is supposed to affect.
I am speculating that either the red-blue dial lacks an electrical connection to some sort of blend air doorway stepper motor (the Mixing Actuator: Bentley's Page ELE -246?) or else the blend door unit is not functioning properly.

I read several posts of others experiencing the same issue, yet I have not read their resolution.

In some posts, folks believed that the red-blue dial was useless.
That must be due to misunderstanding of the red-blue dial system.

In other posts, folks who did not get hot air out of the center vents were instructed to air bleed the radiator.
This appeared to solve the issue for some folks; but, it was not stated that these same folks also lacked hot defrost and also lacked hot floor ducts.
I feel that the distinction of having super hot Defrost and super hot Floor ducts is critical to differentiate an air pocket issue vs. a temperature blend door issue.
I feel that heat source is available, but the fanned air is not going down the correct path.

Can the experts educate me?