For whatever reason, whether shooting S-Log3 internally or ProRes RAW via the Ninja V, Rec. 2020 PQ colors are far less accurate than Rec. 709. In the image below, we can see that the color chips of the X-Rite ColorChecker Passport Video line up almost perfectly on the vectorscope in Final Cut Pro in rec. 709.
On the other hand, the same color chips are wildly off in Rec. 2020 PQ, with a decidedly heavy green shift. Skin tones also deviate further from the skin tone line of the vectorscope.
Using Hue versus Hue in the HSL curves of Final Cut Pro, we can correct for the green cast. We can also save this as a preset, since this correction will be needed on all of our HDR footage shot with the a7s III from here on out.
And here is what the vectorscope looks like with our correction. The skin tones will still require some work!