I think it's either the YouTube TV app not getting correct information from webOS about resolutions and capabilities available to it, or vice-versa. I view the same content through my HDMI connected Apple TV 4K YouTube TV app, and it does get the the 2160p feed, instead of the 1080pX2.
This is the YouTube TV app on WebOS with a qned8500AQ series LG:
while this is. on the same TV, but through an Apple TV 4K via HDMI:
Unfortunately, fixing this probably requires some sort of cooperation between corporations, so I'm not holding my breath for a fix.
