Apple TV 4K bandwidth throttling and bitrate

I've noticed in the past few months that 4K movies on my Apple TV 4K system don't always have as a good a picture as I'd expect.

I live in a rural part of the UK with broadband speed limited to around 35mbps, so not ideal but it should be adequate for 4K streaming. The Apple TV is connected via a short cat 5e network cable directly to the router. I activated the developer options on the Apple TV so I could see the bitrate etc for movies, and this gives some confusing results.

Watching a 4K film via Apple movies e.g. Mission Impossible Fallout on a 4K television, the Apple TV recognised the 3840x2160 resolution of my tv, but the 'natural size' - the actual resolution of the movie given is 1920x1014.

Network bandwidth is given as only 12 Mbps although when I run a broadband speed test at the same time this consistently gives a result of 29 Mbps, and Apple TV reports the average bitrate as being only 10 Mbps.

Watching 'Ad Astra' another film labelled as 4K gives similar results.

Switching to a third movie, 'Logan' the network bandwidth and bitrate stay approximately the same but the natural resolution goes to 3840x1600, although the image quality isn't visibly better.

Using the Amazon app via the Apple TV to watch another movie, '1917' it shows the network bandwidth as 32 Mbps, and natural resolution as 3840x1600, average bitrate still 10 Mbps, but a visible improvement in image quality.

So it looks as if not all movies advertised as being 4K are 4K, and that Apple are possibly limiting the bitrate. Has anyone else noticed similar issues? I'm aware that some bandwidth limitations were applied in Europe due to the Covid pandemic, is this likely to be the cause?