Mixer/Blend shaft controlls the ratio of warm to cool air not the vent allocation.
Remove the glovebox and ave a look in the Passenger side foot wheel up under the dash next to the icc and you see the HIM , its a white/tan coloured unit with three coloured levers coming out of it.
Have a look and see if and of the link rods have come off the levers or if the levers actually move when changing vent assignment.
Also try disconnecting and reconnecting the him. I've seem them come back to life after doing this.
Or get really techo and use Forscan and an ELM327 to run the HIM test/diag