HVAC heating and cooling circuits
Your thermostat repurposes W1 (call for first stage heat) as O/B (reversing valve) when in heat pump mode, instead of having a separate O/B terminal that's unused in conventional mode and using W1 to control the emergency/auxiliary heat. (Instead, W2 (call for second stage heat) is repurposes as AUX/E (auxiliary/emergency heat) when in heat pump mode.)

So, what you are seeing is absolutely normal in your case, merely peculiar to your model of thermostat.  See page 6/PDF page 8 of the manual for details.