As far as I can tell from research, it's best to leave it at Auto or 2160 RGB. That way for non HDR games it will stay at 2160 RGB and for HDR games it will switch to YUV 422.
The reason it switches between the two is because HDR requires 10 bit displays, but as far as I can tell 10 bit RGB isn't supported by current HDMI standards (at least at 4k/60fps with HDR, which requires around 23 gbps of bandwith). So for non HDR games you would want it to be on RGB, and for HDR games like Horizon you would want it to switch to YUV 422 in order for HDR to work. It sounds like your TV was already correctly switching between the two, so I would leave it set to automatic, which is what I been doing with my TV as well. I can say I had some fun as well trying to figure out how and why I had to set my TV's HDMI ports to be enhanced, you would think that would be the default setting, but yeah this has to do with HDR.
Looking around for comprehensive lists I think this is one of the best -
https://www.resetera.com/threads/all-games-with-ps4-pro-enhancements.3101/That being said you shouldn't have to worry about it too much, your system and TV should be able to detect right away whether or not the game supports HDR automatically. That way you don't have to calibrate the settings on a game by game basis.