FCPX and CineForm, what's the trick?

Background: I've been rendering GoPro Fusion files in CineForm and editing in FCPX for the last couple of months. I've since learned (after buying a new Mac) that this is a bad idea, and I should be rendering in ProRes 422 instead. So moving forward, easy enough, I can continue to edit Fusion 360 files on FCPX.

However, I have a bunch of older stuff that for (whatever reason) worked just fine in CineForm on FCPX, but it does not work on my new install on FCPX (it says plugin is missing). I only started using FCPX (10.4) in December, haven't installed plugins, and I'm really new to this entire world. I have installed plenty of Codecs in my old computer, which I've since re-installed on the new system, but still haven't been able to edit the old FCPX files.

TD;DR Simply put, the new iMac will not import CineForm files, while the old one does, and I have no idea why. But I do have files rendered in CineForm that I'd like to edit on the new Mac, and can't.