Hey Dan,
I know this isn’t advice for your problem specifically, but it’s worth trying since it’s a really odd issue.
I’ve had a couple weird FCPX problems that I couldn’t find duplicates for online, and that nobody here had an answer for, that were fixed by creating a new user account on my Mac. Nothing else worked; not even deleting/reinstalling the app, updating, or deleting PLIST files. But a running the operation under new user account resolved them.
I mean, I’d wait a couple days to see if this is a known issue, but barring that…