Retroactive - yes. Ex post facto has a specific legal definition and, if that definition is used, it does not meet the criteria. It's still dirty pool and an underhanded shot.
The real goal was to do as much as they could to take down high cap magazines and AWs without triggering confiscation. When I talked to Linskey (encounterd him on a sidewalk - he didn't have a protective detail so at least he was "walking the walk" of being personally unarmed) he told me the big issue for the conference committee was "how to deal with high cap magazines".
This creates a pricing quandry for dealers - raise prices because of last minute buying, or lower them to clear out inventory while they can.
Lol given reality and no realistic way to regulate sales, that ban is completely unenforceable.