00 Altima coolant issue

[rn240sx]

So my nephew ran into a leaking upper radiator hose… As i went to change it out i noticed how dirty and rusty the coolant was… Popped the radiator cap off and saw the fins and how rusted and bad they were…
We changed out the radiator (after flushing the old radiator and motor several times 1st), put in a new cap and new resevoir (cause it had a whole in it)..
Now for some reason after a drive & cool down, he notices that the resevior tank is MORE full than when started at the MAX line… and now the coolant level from the radiator is lower than b4 cause its apparent the coolant has escaped the radiator and into the resevior but never went back to the radiator upon cool down…
We took the radiator cap back to auto zone and got a new one…. still the same problem.. the upper radiator hose is NEW, the radiator is NEW, the resevior is NEW and NEW cap… lower radiator hose is the same…
NO Leaks anywhere, not overheating… So why is the coolant NOT getting sucked back into the radiator after the cool down period..?? After a few drives here and there the resevior tank continues to fill to the top and radiator just keeps getting lower and lower…
Im stuck here and don't know what else to tell him… I've changed out numerous radiators in numerous cars before and never ran into this problem…