I have been editing 4K media in 1080 timelines in FCPX — just finished a series of 7 vids for a client. I’ve been syncing with PluralEyes before ingest with no issues, but I have experienced a spatial conform bug: you must set to “fit” if you’re going to XML out to Resolve for grading. If you set to “none,” which feels right logically, and then reframe 4K footage (the absolute best part of using 4K in a 1080 timeline), your XML to Resolve will be unusable — the transform specs just don’t make the journey as they should. You must bring into FCPX with spatial conform set to “fit.” Doing that will keep everything in line for a Resolve finish or roundtrip. I know this is not speaking to your sync issue, but expanding just in case your plans are to go out to Resolve.
Rhett Murphy
REDWHOOSH PRODUCTIONS
BALTIMORE, MD