Hi,
This is a known issue with the BMD codec installed. It has been topic of discussion quite often on the bmd forum on Creative Cow (search for ‘Luma shift’ there to learn more). A workaround I would suggest in your case would be to NOT use the FCP 4:2:2 codec but the Blackmagic 8 Bit codec, which is compatible with the FCP 4:2:2 Codec so your editor won’t have to re-render stuff. No luma shift should happen now.
Good luck,
Oskar