HiassofT, Couple of
observations:
I also own a 2019 Shield TV pro, a 2016 era Roku Ultra, the Panasonic 820 UHD player, a 10th gen NUC, a mini Lenovo ThinkCentre with 8th Intel mobile U-series CPU and Xbox Series X. Along with a 65" LG GX OLED TV. For every other device that supports at least HDR10 (the Shield and UHD 820 player and XSX also support Dolby Vision in conjunction with the TV), they all output in BT.2020 (Rec.2020)... at least according to my TV. With the nightly 03/16/2021 build, Kodi does in fact trigger HDR10 as my TV indicates it, but unlike the other devices it doesn't output in BT.2020 as my TV doesn't mention that like all the other sources. As a result the color looks washed out--comparing the shield to the Kodi, it looks more greyish. I looked around briefly to see if I could force it to BT.2020 in some config variable but didn't find anything promising. Imo that's definitely the missing piece to get proper HDR10 support.
It appears that if you enable both HDMI ports that it behaves similarly to Displayport MST, and you are limited to 30hz @ 4K. Perhaps I suggest making the dual audio output something that can be configured for those who don't need it?
Also my setup is somewhat interesting since the GX TV is a very recent purchase made this year and my first 4K display. The rest of my existing gear was actually from 2007 in addition to my receiver that I'm still using it for audio duties in both powering my speakers and decoding audio codecs (currently not interested in expanding my 7.1 speakers to Atmos/DTS:X). I'm pretty sure it's only a HDMI 1.3a receiver. The 4K HDMI passthrough is being handled with both an HDMI 2.0b and an HDMI extractor device. An interesting observation is if I connect the pi 4 directly to my receiver the receiver itself goes a little nutty and starts randomly switching to operational modes I didn't expect. So I had to connect it directly to my HDMI switch. Might be a clue for some folks having issues I'm their setup...
All in all, a great first accomplishment in working to get HDR10 support! The pi4 device is actually serving as a Retro Pi and my Shield is the device I use for Kodi, but I'm more than happy to beta test to assist!