New rules were broken in the past and never punished by God

Leviticus 18:15 says, “Thou shalt not uncover the nakedness of thy daughter in law: she is thy son's wife; thou shalt not uncover her nakedness.”  Isn’t that odd.  I sure am glad this came after Judah did this to his daughter in law.  Why didn’t god know he would make laws against this?  I thought he knew everything.  He must not have known one of his favorites violated this law earlier. Genesis 38:15 says, “When Judah saw her, he thought her to be an harlot; because she had covered her face.

38:16 And he turned unto her by the way, and said, Go to, I pray thee, let me come in unto thee; (for he knew not that she was his daughter in law.) And she said, What wilt thou give me, that thou mayest come in unto me?”  It never says if you don’t know her.  I also notice god says nothing about uncovering the nakedness of Harlots.