Must say the picture through my Xbox one is excellent....Olympiakos certainly seem to be putting on a show!!
I noticed for the first 10 mins of the match there as like a dark filter on all the Ultimate streams. Obviously an issue at their end but all sorted now.
@Aberdeen152what settings do you have in your atv4k box?
You should have 4K 50hz SDR. Then enable match content for frame rate and dynamic range.
@Aberdeen152What happens if you set the ATV4k box to 1080 HDR 50hz and then reload the BT sport app and watch the ultimate stream?
Only other thing I could suggest trying is set atv4k to 4k SDR 50 Hz and disable match content. Just to see what happens.
I just had this exact issue - solved by manually switching the Apple TV to 1080p Dolby Vision - now getting the 1080p HDR stream - completely smooth, no buffering.
I had the same problem as @Aberdeen152 using 4K SDR 50 Hz. The picture kept freezing/buffering, completely unwatchable. I then changed to 1080p HDR 50 Hz and have no problems at all. The picture looks stunning now! Much better than I have ever seen from Sky 4K football, albeit Sky doesn't do HDR yet. The more I think about this, and the more I experiment, I am of the opinion that HDR ("faux" or real as with tonight's match) is making the big step up in picture quality.