I've literally tried it all, and I've found no fixes.
I only listen to music using shuffle, and as of roughly 6 months ago Spotify only shuffles the first ~50 songs in my downloads list.
I've tried deleting and re-installing, I've tried logging out and then back in, I've tried changing the arrangement to list by 'Title' and then changed it back to list by 'Recently Added', made sure that Spotify is updated, and LITERALLY nothing fixes this.
No matter what, Spotify just keeps shuffling my ~50 most recent songs.
I read somewhere that Spotify might be doing this by design to save on server costs, and it's absolutely making the experience unbearable. If I can't find a fix, I'll be (unhappily) jumping ship to Apple Music ASAP.