An easy thing to try when FCP behaves strangely is to delete its Settings. Hold Option and Command while launching FCP and choose Delete Settings. It will open the default Untitled library. Re-open the library you were working on, set the Settings back to the way you like them (disable Background rendering, etc.) and see if the issue has been resolved.
If that doesn't change the behavior, please post the exact specs of the media and what camera was used. The output of Invisor or ffprobe is good.
Is the media stored in a folder associated with iCloud Drive? Perhaps it was enabled during the 15.3 upgrade?
Paste an https://etrecheck.com report using the "Additional text" button below in your reply.