G6PD deficiency is about half-life. The mutated enzyme does not last as long as a typical RBC would live (120 days), so the older RBC's will begin to have a lack of G6PD enzyme that they need to deal with oxidative stresses. So if you check the enzyme levels right after all the old cells have been lysed, you won't see a problem with G6PD because only the newer RBC's (with plenty of still-functional G6PD enzyme) will be left around.