The temp at which the heat pump will continue to operate is controlled by the outdoor coil freeze sensor. The sensor will open at 18deg F and close at 38deg F, (coil temp, not air temp). The tolerance is +/- 3deg F.
Once you get the coils cold enough and the unit shuts off because the freeze sensor opens, then the coil temp needs to raise enough to close the sensor (35 to 41 deg F). If your freeze sensor happens to be one which closes at the lower end (35 deg F), then the unit will operate at lower temps than one which closes at the higher end (41 deg F). The amount of frost on the coil, which relates to humidity, also factors into how long it would take for the coil to reach the sensor setpoint.
If these heatpumps were equipped with defrost capabilities like home units are, they would operate well below 30 deg F.
Now, throw the thermostat into the mix and things can get screwy. In the electric heat mode, you have 2 stage heat. The first is the heat pump (both compressors), the second is the LP furnace. If the heat pump can't maintain the set point temp, the LP kicks in. If this happens 3 times in a row, without a 20 minute period of the heatpump not requiring LP backup, then the thermostat will lock-out the heatpump for 2 hrs, during which time the LP will operate alone as called for by the tstat. After 2 hrs, the system will reset. Now the heatpump will come on, but this time it gets only 1 strike. If it can't maintain set-point temp and LP come on, it is locked out for another 2 hrs. And so on, until the heatpump can satisfy the setpoint, then cycle starts over.
If you are in a 2 hr lockout you may think the heatpump went bad, but not so. I believe, although the documents don't say, that you can reset this lockout manually by turning the termostat off, then on. I think this resets the internal strike counter.