Heatilator GC150 NG - Pilot goes out only after turning the fireplace off

[PAUBAR]

Heatilator GC150 NG - Pilot goes out only after turning the fireplace off.

• When the fireplace is cold I can light the pilot and it will stay lit for days/weeks.
• When I turn on the fireplace and it gets hot, the pilot continues to remain lit.
• When I turn on the fireplace and it gets hot, and then turn it off, the pilot remains lit for 3-4 minutes and then I hear a popping sound and it goes off.

I recently replaced the entire ignition unit and all voltages are normal and switch works fine

Any thoughts on why the pilot may go off only when the fireplace starts to cool after use?