Hi, i've just experienced the same on my Fabia. The flow was correct with recirculation activated and it was fluctuating when it takes fresh air from outside. I fixed it last week by cleanning a little potentiometer with alcool into the recirc flap motor housing (filled with grease so i think some goes into this potentiometer, bad design 😞 ).
I removed the glovesbox. I didn't disconnect the "airbag keyhole" and i've done everything with disconnected battery. You have only two screw to remove the motor housing but it's very hard because of the situation... but it's harder to put it back, there is really no space and you must adjust the position of the housing with two flaps levers ...
Watch this https://cardiagn.com/skoda-fabia-mk3-heating-air-conditionin/ p.70
And you can make a reset of all flaps by pressing the AC button, face button and "Auto" button at the same time with just ignition turned on