Picture quality and Apple TV 4K

Hi,

I bought a Toshiba 50VL5A63DB TV about 7 months ago, and have an Apple TV 4K box. I started watching Apple TV+ on the night it launched, with The Morning Show. In the opening scenes, when it was dark I noticed noise (If that's the right way to say it?) on the screen. I have been trying to sort out my picture since then. During this, I had Dolby Vision on via my Apple TV 4K.

According to my TV specs, it supports Dolby Vision (https://toshiba-tv.com/uk-uk/50vl5a63db) - when I reset the video settings using Apple TV 4K, it chooses 4K HDR, but the problem occurs then too. I tried changing many settings on the TV itself and on my Apple TV 4K box, with not much improvement - until I changed my Apple TV 4K setting to 4K SDR, and suddenly the noise was gone.

I am puzzled. If my TV supports Dolby Vision, why does it struggle to display it correctly on the TV? This does not occur with other 4K content (from iTunes at least). Could it be the HDMI cable I'm using?

Here is the difference in quality between Dolby Vision and 4K SDR. 4K HDR is a bit better than the Dolby Vision image, but it's annoying: