LG WRGB OLEDs work in REC.2020 in HDR mode; they don't have a P3D65 HDR mode.
LG has a hidden menu where the users can override the input signal using the HDMI Signal Override. (highlight the picture mode in the main menu and press ''1113111'' from the TV remote)
That hidden menu has the option to define the working colorspace, EOTF, and the ST2086 Metadata (Mastering Monitor Min/Max, colorspace, MaxCLL, or MaxFALL)
While the 'Colorimetry' menu has a P3D65 option, LG has never enabled that, so it's been a cosmetic option for several years.
When you set 'Colorimetry' as P3D65, the TV will still work in REC.2020 mode.
The other 'Colorimetry' options work, like REC.709, REC.2020, etc.
Since the TV doesn't have P3D65 mode, it's introducing an issue when content creators (home/studio) use it as a client view monitor with P3D65 HDR as a working colorspace.
It's a problem for home/small studios users who use the LG WRGB OLED as their only HDR monitor.
If you grade using another monitor which uses a calibrated P3D65 HDR mode and playback it to the LG OLED, the 100% P3 Red (for example) will be displayed as 100% RED in 2020 to the LG; colors will clip, and the whole presentation will look wrong to the LG.
Basically, LG has a 3x3 matrix before the internal LUTs, and it's available a special workflow for Netflix in Calman Ultimate to upload a 3x3 matrix upload which converts REC2020 PQ to P3D65 PQ.
However, you use a Technical LUT to convert REC.2020 PQ to P3D65 PQ.
That Technical 3D LUT will map the P3D65 inside the REC2020 so that you can send P3D65 HDR signal to the LG without color.
You can use ColourSpace or LightSpace to create that Technical LUT, select the target/source gamuts as you see below, and generate the LUT.
When using an LG WRGB OLED as the only HDR monitor, you can use Resolve and load that Technical 3D LUT as a 'Video Monitor Look Up Table' (tetrahedral interpolation).
In that case, export the 3D LUT by selecting the 65-Point Cube BMD 3D LUT format.
Resolve 18 allows the capability to have an active 'Video Monitor Look Up Table' when the Dolby Vision is enabled (it was disabled that function in Resolve 16/17).
When you have disabled the Dobly Vision, the 'Video Monitor Look Up Table' will work with any Resolve (16/17/18, etc).
When you have two monitors in the same setup, one calibrated for P3D65 PQ and the LG WRGB OLED, you have to use an external solution to load that Technical 3D LUT for the LG WRGB OLED.
A low cost solution with HDMI 2.0 input/output can be the BMD Bi-Converter 12G, its supports 33-Point Cube 3D LUT, so export 33-Point BMD 3D LUT from ColourSpace.
Any other 33-Point Cube LUT Box will work also.
You can use the HDMI Override to set colorimetry to REC.2020 and ST.2084 when using that Technical LUT.
If you want to disable the TV's roll-off to force clipping to the LG's calibrated nits in HDR mode, you can use the LG PQ Upload Template (2019-2022)
and upload 100 value to 1/2/3 roll-off points.
Using 100%, the TV will follow the PQ and not roll off up to its peak calibrated nits.