Thanks for the responses.
It looks like everything is set correctly in FCPX. I have done some quick tests and have noticed that along with the wiggly line the entire image quality is lowered as soon as you crop 2%… It actually looks like a low-level gauze or soft-focus filtre. Unselect crop in inspector and the clarity is back and the lines are not wiggly…
The original file that made me catch this was exported from within FCPX, using the «export using compressor/YouTube 720p HD» setting. The same degradation of the file within FCPX (post-cropping) did come through in the final output file, unfortunately.
I am now doing tests to see if this holds true on export for other settings.