I just completed my first manual HDR calibration on my LG OLED CX. As for the 2.2 gamma calibration, I think it went pretty well for a first attempt...



However, after switching to the ST2084 Rec2020 (733Y) gamut to check the final result, something must have gone wrong because the diff EOTF looks completely different compared to UHD Rec2020 with 2.2 gamma.



seems set up correctly to me...



Am I missing something?

What you're seeing on the ST2084 diff EOTF graph is a result of the TV's tone mapping being applied. The set cannot approach the fixed luminance requirements of ST2084 above 60-70% stimulus so tone mapping is used to compress those values into the luminance range the set can actually produce. In the "RGB with Luminance" graphs a ColourSpace competitor produces, you see a similar inverted "shark fin" at roughly 70% stimulus. It's normal when evaluating an HDR calibration.