The issue is that, legally speaking, Ex Post Facto doesn't apply here.
If something is declared contraband, you are prohibited (or restricted) from possession moving forward.
Cocaine used to be legal, right until it wasn't.
Alcohol was legal, then prohibition happened.
Bump stocks were legal until they weren't, now they might be again.
Ex Post Facto would be make things illegal today and prosecute you for owning it 5 years ago.
It is also why you don't have to tear apart a home every time they change the building codes. Things were done correctly and in accordance with the code at the time the home was built. When you make changes you might have to update the affected areas you are working in, like making sure a bathroom has GFCI outlets, but you typically don't have to do a to the studs rebuild to the whole house just because you update your bathroom.