We've found the image quality on Amazon Prime to be wildly variable. Their 'top tier' productions such as Rings of Power tend to look fantastic, especially with Dolby Vision with the £2.99 ad-skipping add on. Other shows, particularly older HD or below show, can look absolutely awful. We started watching Lost with our son prior to Disney+ launching; the night time scenes had horrible colour banding, blacks becoming reds and compression artifacts everywhere. Scenes often looked like shifting Lego blocks.
I did watch Immaculate on Prime a week or so ago, a very recent film. Once again, shifting Lego blocks, odd/inappropriate colour bands, the works. Every other service - Sky, Apple TV+, Disney+, Paramount+, Netflix etc all looked absolutely fine.
Muy own suspicion is that Prime often use too low a bit rate for their video compression codec, resulting in the artifacts and incorrect colours.
Sky Glass & Stream Puck customer w/Sky Entertainments & Netflix, Sky Cinema, Whole Home & UHD/Dolby Atmos add-on.
