Blend door problems cause temperature-control issues. Vacuum problems cause airflow-direction issues.

Sounds like you have a vacuum leak somewhere. If the airflow defaults to the defrost vents only when in MAX A/C mode, then it could be a leak in that part of the HVAC system that is associated with (1) the recirculation door, and/or (2) the heater control valve, both of which are activated (closed) when MAX A/C is selected.

See diagram HERE.

It's a good idea to pull out the mode selector switch and check the hose connections in back of the switch. Take note of the white vacuum hose, in particular, and where it goes.

You also want to check the gray hose that tees off the white hose and runs out to the heater control valve located in the engine compartment.