I'm finding that the first frames of a fade out to black are brighter than the preceding part of the outgoing shot, then the fade proceeds to black. It's as if it doesn't start by dissolving to black but to a paler underlying tone.
I have also tried using motion tabs to alter the opacity and tried cutting in some slug and cross fading but both had the same result.
On fades up from black I get the same effect - the last frame of the fade is brighter than the first frame of the incoming shot.
This is with Avid DNxHD codec material at 1920x1080
I don't remember this being an issue with earlier versions of FCP.
I'd be grateful if anyone has any clues what to try next.