I got this film I color corrected using FCPX plugins and the color board and it came back from QC with RGB Gamut Errors. Went to my timeline and sure enough, on a lot of the shots the setup levels were often crushed, often to -10 and the and the gain was well over 100! I know I didn't correct it that way so I did some tests. It turns out FCPX is looses my correction during the render! I thought may be the render files were corrupt so I trashed my renders and all my levels came into spec. I re-rendered and it was crazy again. I can't get my correction within spec. Any ideas?
Try exporting the timeline without rendering to see if the corrections look to have stayed. FCPX renders automatically upon export but will not create render files that will be retained after export in your timeline. If the exported file has corrections that seem OK, start FCPX while holding the Option and Command keys and when presented with the dialogue to delete preferences, choose OK. Then reload your project and check that the levels are fine (since you did not render anything from the prior step), and then render the timeline - see if it is now fixed.