I have a aux heater that kicks in when the heat pump can't keep up, and I have noticed this. When the heat pump is running, the air coming out of the vents is barely warmer than ambient. Definitely less than 70F. When the aux heater kicks in, the air is actually hot.

A side effect is that the heat pump needs to run a lot, because heating a house to 62F takes a long time with air that it 65F.