So I upgraded yesterday on my imac Pro, and notice that the waveform has changed in rec 2020 wide gamut library to include up to 4k. Cool.

Issue is that now my prores footage that was properly exposed on an Atomos Ninja Inferno and Ninja V (multicam) now has blown out highlights. They are not clipping in the waveform...not even close, but grey hair for example, is zero color information there. The practical lights in the background facing the camera are not, and still in range, even when using a rec 709 conversion LUT, but the grey hair of the subject is. I exposed the images and know they were will within range for V-Gamut.

Any ideas?

What are you actually viewing the footage on? The iMac Pro or an external display capable of wide gamut and HDR? If just on the iMac, using 10.4.7 with Catalina enables tone mapping so you're supposed to be able to "reasonably" see your wide gamut HDR footage on a regular display without puking. But I doubt that it's perfect, and it's not supposed to be a substitute for a proper display for grading. It's only meant for you to say, 'wow that's not completely messed up.'
Finally, I would beware of something like a rec 709 conversion LUT... I would only use something like that for rough viewing purposes. Depending on what it's used on, it can clamp down hard on highlights and kill shadow detail. Hard to say because you don't say what format is your footage (my guess is some kind of log to some kind of ProRes since you're using the Atomos).

Thanks for the response. I'm using V-Log L which is basically V-Gamut and I have set as V Gamut on the Atomos. The problem is, FCPC is STILL importing V-Log footage as rec 709, even in a wide gamut library. I had forgotten about that, so you have to use color space override to change all the clips to Rec2020 PQ. That makes a massive difference in how the clips are displayed and the waveform displays correctly; however, I still have clipped highlights on the iMac Pro display, even though they are not clipped on the Atomos or waveform.

Yeah, it will auto-apply certain LUTs on clips to bring it to Rec 709 at import depending on what it detects. It's meant to do that so you can quickly preview footage in a linear color space, and I suppose it doesn't move it to wide gamut because people are unlikely to be able to preview it on a wide gamut display... either that or they just overlooked it. Personally, I think it's an annoying feature, but it takes a few seconds to change it so I don't care too much. There should be a checkbox in the import window or in preferences to turn this off, though.

The Inferno as far as I know can display HDR, so that's why you'd be seeing it properly. On the iMac Pro you'll be getting the tone mapping which is throwing away tons of data to push it into a standard dynamic range. Remember that the iMac Pro display has expanded color, but it's not HDR. I mean, it could be a bug with how the tone mapping is happening, too, so it's hard for me to say without seeing it all.