So I had 4K 120Hz with chroma subsampling running for a while now, even before upgrading my GPU to a 6800 XT. No Freesync there, screen would flicker or display no signal.

Upon getting the 6800XT though, I would retry to get Freesync working and it also wouldn’t do it - kinda. When I set it to 4k120 10bit 4:2:0 and enable Freesync Ultimate it would display Freesync in the TV OSD but it also stayed at 120Hz at all times and very obviously did not work in games.

Then one day it would work all of a sudden and I wondered why.

Turns out, there are a couple of things that need to happen:

Connect PC to TV HDMI Port 4
PC - Boot Windows
TV - Turn on
TV - Enable Input Signal Plus on Port 4
TV - Enable Game Mode, Freesync Ultimate, disable Motion Interpolation etc
Enable Freesync Ultimate
PC - Set resolution to 4k 120Hz (not 119Hz) in Windows Display Settings, this should also engage 4:2:0 since it wouldn't be possible otherwise, disable HDR (not sure if this is necessary)
PC - Set Color Depth to 8 bpc in Radeon Software and Pixel Format 4:2:0 if not already happened
PC - Shutdown
TV - Turn off
PC - Boot Windows
Wait for it
TV - Turn on
Fire up a game, ideally something that will hover around the 110-120fps mark in 4K
Press the select button on your TV remote to display the OSD and check if the Hz change rapidly, that means it's working.

I made a quick video with my phone how it should look when working:

https://imgur.com/a/qgk4skD

My setup:

AMD Ryzen 5800 XT
XFX Radeon RX 6800 XT MERC 319
MSI B550 VISION D
16GB RAM
Windows 10 19042.746
Radeon Software 20.12.2
Club3D CAC-1372 2m HDMI 2.1 Cable
Samsung Q90R Firmware 1374.0