> My prime conjecture now is that there's some kind of caching reason where it's more advantageous for their CDN if those handful of songs are the only ones that're played.

It's far more sinister than this. It has to do with royalties. They've got some secret algorithm that will even cut your account off from specific content if it's expensive and you consume it too frequently.

I find no issues listening to pop cult shit 24/7/365, but when I want to listen to some obscure opera more than 3 times it inevitably starts to fade out like a ghost.