I keep thinking that's a bug (about no NTG awarded with 5*), but apparently it's probably just bad luck. It's not 100% chance of NTGs even with 5 star I think. That's why we have the Uni upgrades for higher x/y NTG chance, and the Museum artifacts for higher NTG chance. It would be good to know the actual % value though... so that we can know how far to upgrade artifacts for a 100% chance.