I have seen this even in the early 4k streaming periods. Netflix and Amazon were both pumping 25 mbps. It looked great in static scenes until there's a lot of motion or classic codec kryptonite like confetti. Overall pretty good though.
A UHD bluray is also compressed but runs between 72 and 144 mbps by comparison. Between 3x and 6x compression ratio to the internet streams of 5 years ago.
Then came the realisation that bandwidth costs money, and if you reduce it per customer, you do better business. The 4k streams got recompressed and stepped down to 15 mbps and later to 10 mbps. HBO max seems to pump 8-11 mbps, so same ballpark. Now we're talking about a 7-10x ratio between disc and steam.
Lossy compression is not free, you lose something in the process. I don't care how much compute and machine learning you throw at it, you can't make a video compress that much without issues. You certainly wouldn't expect to not notice issues in a 120kbps mp3 versus FLAC. They hope you don't notice what is missing because it tends to be in low contrast areas, and in static parts of the image. As a result you can literally see more detail when nothing is moving on screen. Sadly we are watching these things called moving pictures.
A good 1080p bluray often works better (for me) than a 4k stream. It makes a big difference to moving scenes and you don't get blockovision moments when the adaptive codec has a panic about the available bandwidth. I hate that our internet capacity improves but we're not given the option to pull more data.