Not uncommon for straight heat pump settings. Usually the backup heat strips provide the most heat output when activated and even if they are installed. The highest temp I ever got from a straight heat pump was 92 degrees.
Measuring the temperature of air at the registers is always deceptive. The goal is to bring the ROOM temperature up to the thermostat setting. Register air quickly dissipates and cools as it mixes with cooler air in the room. A Home Inspector needs to check that each habitable room has a heat source and that it operates (supplies warm air). Any attempt to measure the temperature or efficiency of the heating or cooling will be misleading, time consuming and way beyond the standards.
The newer units/thermostats are somewhat tricky to bypass the unit’s brains that are programed for most energy efficient operation. From talking to some HVAC guys you may not be able to get the Heat Strips (or gas heat) to come on using “Normal Controls”* if the outside temp is above a certain temperature (something like 45-50). You may need to really know what you are doing to bypass the normal operation of the Thermostat for Aux-heat/heat strips to kick on (same with some those that use Gas for Aux heat mode. I just really shy away from trying to bypass anything which could screw up the programming. (Kind of like trying to check sprinkler timers, so many different ones out there)
(* we should probably not be bypassing and going into an expert or Diagnostic mode at the thermostat)
I love the old double bulb mercury thermostats where you can easily see when it is operating in Heat-Pump versus Aux- Heat mode.
This is my understanding, when there is an auxiliary (or emergency) heat setting on the thermostat, you must manually switch it over from heat-pump to auxiliary heat. This is because a heat-pump will be unable to produce effective heat on a really cold day (around 20 f or less?). And, not every heat-pump has auxiliary heat. This is often (in warmer climates) an added accessory.