I believe ex post facto in the US only applies to criminal cases. Retroactive civil regulations are not restricted (at least by the federal constitution).

https://www.oyez.org/cases/1789-1850/3us386

Correct:

In a legal context, ex post facto is most typically used to refer to a criminal statute that punishes actions retroactively, thereby criminalizing conduct that was legal when originally performed. Two clauses in the United States Constitution prohibit ex post facto laws: Article 1, § 9 --- This prohibits Congress from passing any laws which apply ex post facto. Article 1 § 10. --- This prohibits the states from passing any laws which apply ex post facto.

<https://www.law.cornell.edu/wex/ex_post_facto>

There are laws that make contract clauses illegal/unenforceable. Think right to work.

It’s different from ex post facto, but

The state can not pass laws that alter existing contracts or refuse to enforce them. That’s very explicitly prohibited under the contracts clause.

Very much unconstitutional

> The state can not pass laws that alter existing contracts or refuse to enforce them.

"Right to work" laws would seem to be a counterexample here - states are permitted to jump in and put their thumbs on the scale in a contract between a debtor (the employer) and the union.