Because you said, " I thought compression was in the coolant but no coolant missing,no overheating,no coolant in the oil." That indicated everything is working fine.
The coolant system works under pressure, by opening the cap while engine is HOT , you release that pressure and coolant/water comes out immediately. When you put water back in and close the cap, there is air in the system which will self bleed if you don't constantly opening a hot coolant system.
Did you use 50% coolant/50% water? Sounds there is nothing to be concern.
