2019 Subaru Crosstrek Heater issue
When in the floor only position, max heat, max fan, low volume air output from the floor vents. Sounds like air is coming out the defrost vents but isn't. Mechanical HVAC. Cabin filter is new.
Was this problem there before the cabin filter was changed ? If not, remove filter and probe area with a boroscope. Sounds like something may be inside the heater box blocking air flow.
Problem existed before replacement of filter. No signs or debris in housing.