Maybe I can explain it a little better.

Hairless is it's own gene and it is recessive. The hets do not have any special look to them.
Most rat colors and genes are recessive

Mock hairless, or patchwork, or double rex is a super form of rex. Rex are the rats with curly or wavy hair. The rex gene is a dom (co-dom?), and has no hets.

What I am guessing is one of your females is a poor example of rex. The females do not hold the curly coat as well as the males. Sometimes the only way you can tell a poor rex is the curl in their whiskers.
You are able to see it in the babies because it's just expressed better in pups and then they molt and may lose some curl.