If you think it was just in defrost since it was warm. It's been 48hrs since then, go back and have a look again at the frost pattern.
I too got bit by the "I ran my steamer in the return vent so I know it's clear" issue. Even though you think it's clear and you have a good amount of steam coming out of the other end it could still be particially blocked or partly collapsed. Mine was the home owner used a hair dryer to melt the frost and deformed the return vent. Couldn't see it but it kept returning.
If you verify that the frost pattern is good. You are looking at air flow. That boils down to return vent blocked/collapsed or reduced RPM evap fan motor, or damper(though I think yours is not mechanical, user sets the damper).