DIAGNOSIS The wiring configuration is incorrect, or the 'O/B' setting for the heat pump reversing valve is wrong. This causes the system to activate in cooling mode.
FIX Verify your system type during setup. Remove the display. Check the wire in the 'O/B' terminal. Go to Settings > Equipment > Heat Pump. The 'O/B' setting must match your system: 'O' energizes in cool (most brands), 'B' energizes in heat (some brands like Rheem). Toggle this setting.
IF THAT DIDN'T WORK The wires for 'Y1' (cooling) and 'W1' (heating) may be swapped at the Nest base or the HVAC control board. Trace the wires and confirm their correct terminal placement.
HEADS UP If you have a conventional gas furnace with central AC, this symptom indicates the 'Y1' wire is active when it should not be. Check for a wiring short between Rh and Y1.