I noticed that same issue a while back. After checking the vent controls, fluid levels, I decided to change my thermostat bc of a thermostatic fault error and it solved my heating problem.
'07 Audi A8 (w/ 20,000 miles)
'03 745Li (sold)
'03 Acura TL-S
'02 Infiniti QX4 (sold & new owner totaled it 4 hours later)