FWIW, the precedent distinguishing prohibited ex post facto laws from permitted retrospective laws was established by the Supreme Court when ink on the Constitution was practically still wet, in Calder v. Bull (1798).
I think there is an applicable quote about defending scoundrels that would apply here.
https://en.wikipedia.org/wiki/Smith_v._Doe
An interesting side note is that many state courts have continued to strike down similar provisions, and some Federal courts have apparently also concluded that certain rules about sex offenders are "punitive" and so constitutionally invalid as outside of the Smith v. Doe rule. So this example is hard to summarize in a simple way, not least because many other courts have been trying to narrow this rule.