Thanks @flx. Yes, you are correct. The Reservoir begins to get full when the engine gets hotter.
Thats Normal
Thermal Expansion , applies pressure on the RAD cap
at 0.9 it is spec's to the spring will release coolant into the overflow
if your getting more coolant then normal coming out and filling a OEM capacity overflow to max and beyond ?
then I KNOW exactly why this is occurring , there is ONLY one possible reason .
is this your exact or close scenario > going on