@kletter1mann The same culling happens on Amazon/Prime Music including the most expensive HD Family plan that I switched to from Tidal. And it can be very frustrating when tracks suddenly disappear from custom playlists. Not only that, but on AmazonMusic, they will also change-up the quality offering... Let's say you added the UHD (best sound quality version) to a playlist, then they change the SQ to HD - this causes the UHD version to become unavailable. So the only way to get it back is the search for it again and add it back to the playlist in the lower quality.
The other gripe I have with Amazon Music is when you buy the digital music, it is MP3 - unless you buy the physical CD and rip it when it arrives - thats lame.
My point is Amazon is no better than the rest in this regard - we might have thought the libraries would be ever-increasing, but this is not the case. Instead, they are ever changing, and without warning become unavailable or downgraded. I've seen tracks disappear, then return again a couple weeks later - but usually not.
@mspot has it right... use streaming services for discovery, then buy what you want to keep.