First time I've had this issue or noticed but I have noticed a difference in brightness of a video file inside FCPX after importing. The video file was a BRAW output from DaVinci Resolve as ProRes 422. It is a UHD file on a 1080 timeline. I've checked and I do not have any color correction applied to the clip in FCPX. When I preview the file in quick view or QuickTime the brightness appears to match what I see in DaVinci Resolve. Only FCPX shows the brightness change.
Well, I was wrong. After further comparing the file inside DaVinci Resolve and FCPX they look identical. The only place they look different is in quick view and QuickTime. So, though it would be nice if they looked the same in QuickTime and quick view it's not a problem as long as it looks the same in DaVinci and FCPX.