2019 Subaru Crosstrek Heater issue

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.

on hyundai‘s on vents operation when its on floor theres also air coming out for the defrost which is normal. I would suggest to check vent operation in service information. Hope that helps