A couple of months ago i had to replace my heat pump and air handler

this is what i put

heat pump - carrier 25HCC536
AIR HANDLER - carrier fv4cnf005

Anyways recently its been cold and i have the thermostat at 70 and when it starts it blows cold air lowering the temp of the upper floor and then when it reaches 68, on the thermostat it shows aux heat and finally it starts blowing hot air and warming the upper floor.

Called the company that installed it and they look at the gauges and connections and the technician said that he didnt see anything wrong that if would happen again to change the thermostat, could it be really this?