Apple TV 4K - Match Dynamic Range reverts everything to SDR

My HDMI cables, TV settings all work just fine when the matching is turned off, content plays fine.

Is that with the base setting set to SDR? I.e. content plays fine, but not HDR?

Do you use a receiver, or a direct connection?

Some TVs and receiver have a few select HDMI ports that support 4K HDR, while the other ports are not top tier.

Some HDMI cables do support 4K, but are not certified for 4K 12 bit HDR.

If the HDR matching test on the Apple TV fails, then it will stream SDR content.

I don't believe it's anything to so with the Apple TV so I suspect restoring all the time is a likely waste of time (especially if you have a lot of apps to re-install. I have seen others report this issue, but suspect it to be specific to 3rd party equipment such as TV's, cabling, receivers etc. You might try it with other equipment, elsewhere, but it may be something you need to live with. I'm surprised you have the problem with the LG though, I don't have the same model but my set up works just fine with the LG I do have. (It appears, from monitoring these communities that the most troublesome TV's are from Samsung)

All a cable does is route data, the only difference between one cable and another is how quickly they can route that data. It is the cards at each end (where the cable plugs into) that determine what features are supported, however DV and HLG is incorporated into the video signal and as such it doesn't actually matter what HDMI or HDCP specification is available at each end (assuming 1.4 or later), HDR10 requires HDMI 2.0a.

As for HDCP, again it isn't cables but rather devices that are compliant and the fact that yours works at all demonstrates that your equipment is indeed HDCP compliant.

The Ultra High speed cable sold by Apple is overkill for what is required for the Apple TV, that being said overkill shouldn't be a problem. Unless the cable you have is damaged in some way it should be fine with the Apple TV.

Personally I suspect that automatic switching is possibly a problem with some 3rd party equipment, I suspect that Apple now this and may be why it wasn't offered originally and is now only offered as an option.

Quickstart is on by default, so not really something to fix, rather something to leave alone when the consequences aren't known.

But perhaps the protocol for establishing a connection doesn't allow for the time it takes to preload a movie. Apple doesn't create those protocols.