Guys, from my experience, the Luma shift is a known bug in FCP , not a BMD issue.
The problem is with the Range Check overlay (Control Z) … best bet is to turn it off.
Outrageous really … a tool specifically included to indicate bad levels actually causing your levels to shift if you turn it on!
I’m sure no-one need telling but … use proper scopes when you’re color correcting.
Hope it helps
Andy