I cannot say that I’ve noticed air coming out of the feet, but I’ve only ever had reasonably thick trainers on, so wouldn’t necesserily tell if there was a little, but I assume that it is little different to the feet only setting?
I would look at the linkage between the heater controls and the rest of the system first. You might be able to look through the glovebox area to see what is going on (or not).
Some guys have noticed the pipes coming off the heater box as coming loose. On my Mk 1 Roadster the heat from the vents is very hot and also to the footwells----