Alright I figured it out. There is a bug but it’s very minor.
Basically if you have 486p@24 anamorphic footage, and you have Best Performance set for playback, FCPX thinks it’s interlaced as it scales it and the image bounces up and down in the Viewer, even if the clip is set as progressive in the Inspector. I thought it was exporting like this to Vimeo but I don’t have Vimeo plus so I can’t try and replicate it easily.
By running the source clip through Compressor and cropping off 6 lines, the new 480p version I made does not have this problem at all. FCPX’s Best Performance plays it like it does everything else.