“Quicktime gamma shift ” is a misnomer for people who don’t know what they’re doing. 😉
I have perfectly consistent and valid results by using the correct data type, and container, and making critical evaluation on an external monitor that has been properly calibrated.
The fact is when you’re playing any sort of media in a OS environment there are a huge number of variables as to how that material will be displayed. If any of those are off the result will be inconsistent (as was the case with the OP). By using baseband video you bypass most of those OS and software gotchas. But QuickTime isn’t inherently any worse at this than other media containers, in fact thanks to ColorSync it’s often better.
The OP was monitoring with data levels in an environment that probably expected video levels, when he then rendered that out and brought it into fcpx he got lifted results. As he was clipping on output prior. That, btw, is not a “gamma shift”, it’s a level scaling misinterpretation.
https://JuanSalvo.com
https://theColourSpace.com