Diagnostics in the hands of a good technician can do more than just spit out a fault code. You can read out temperatures and check the flap motors are working as they should. If you had no fault codes then I would suspect the system thinks it is behaving correctly and responding to temperatures as it should. Data on the compressor operation is also available in diagnostics for those that know what it means. It is more likely that an air con specialist would be more familiar with using diagnostics than a garage looking for an easy fix fault code?
Midician makes a good point, but I'm also wondering if the heater matrix is getting hot and giving out hot air? Additionaly, temperature sensors may have shifted and both will appear 'normal' and not produce faults. I also wonder if the flaps are automatically closed if the air temperature entering the cabin is below outside temperature (no heat) to stop screen fogging