Could be something with the electronics of the unit itself (thermostat, wiring etc.). I believe the HVAC system is a smart system as well and will set DTCs if there is something wrong, you could try to pull those codes.
I know a number of people have had problems with the blend door actuators, which certainly could cause no heat to the passenger side. That is what I would check first.
The passenger side blend door is pretty easy to gain access to through the glove box.