Nope....it's not normal.
EDIT: I just bled the vacuum from my selector setting by turning the selector around to different settings with the engine off.
With the fan on, no matter were I put the selector, it only came out of the windscreen vent.
So without vacuum, or lack of it in the system, the windscreen vent is the "default" setting.
So yes, vacuum problem. Suggest what accentstencil suggests first. That little hose from the manifold, goes through the fire wall just under the heater hoses.
If that doesn't work, try what I suggested.
Check the little vacuum manifold at the back of the heater controls. (Means pulling the dash facia off)
It's connected to the selector knob.....Feet, windscreen, face etc.
Failing that, the selector it's self may not be separating the selections properly and when you put your foot down, you are producing a higher vacuum for a moment and the air may bleed into the windscreen selection which incidentally, is the first selection from the right on the VT.
Does your air selector have resistance from one setting to another. It should have, as this separates where the air is going. Feet face etc.
If it just turns from one selection to the other easily and without resistance, there may not be any separation between the modes. Hence your problem.
Failing that.....As accent says ^^^.
