I believe the prohibition on ex post facto laws applies only to criminalization of behavior that was previously lawful. That is, it would not apply to civil matters of any sort, and it also would not apply where immunity is effectively conferred—only where prior actions are made illegal.
Upon quick searching, it looks like scholars have debated the civil/criminal point:
Upon quick searching, it looks like scholars have debated the civil/criminal point:
https://www.quora.com/Does-ex-post-facto-apply-to-civil-and-...
https://en.wikipedia.org/wiki/Ex_post_facto_law#United_State...