This happens with the XAVC-L internal UHD recorded footage. The FS7 XAVC-I does not do this. There must be some meta tag that is telling the SW how to interpret the footage.
The clipping is real at least on 2 of my macs with latest FCPX and Catalyst 2.0.1. I did some more tests yesterday recording both to the Odyssey 7Q+ and internally. I then put the 2 videos on a split screen and it is very obvious on the WF in FCPX and external scopes on a SDI out. I have been searching for a setting in FCPX but I can't find one to fix this. The interesting thing is that the exact same shot in HD (same PP etc) shows all the way up to 109ire in FCPX, but the UHD footage does not it clips at 100ire when rendered, before rendering it actually shows all the way to 109 but if you pause then it stays up to 109 for a second or so then clips it down, very annoying. I will contact Sony about this after the holidays.
You can only see this on a Eval monitor and the FCPX WF as the Mac screen clips to safe levels at all times.
Catalyst Browse clips the footage as well. My OS is 10.10.5
One way around it in the short term is to use a PP like Cine2 that only records up to 100 ire. It does it in all the PPs that record up to 109. Slog and Cine 2 are the ones that it makes no difference on because they all record lower.
This reminds me of problems we had years ago that were solved in FCPX.
I just had a long talk with Apple FCPX tech support a couple layers up.
And they will be fixing this at some point. The FS5 is not on the approved list yet but it will be hopefully soon. Thy seem to think like I do there is a meta tag in the file QT is not reading correctly. So we are waiting for an update.