Disclaimer: I'm a backend person with an LLM. Please forgive my UI/UX, and lack of proper mobile support.
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For the curious, here is a brief explanation of this annoying phenomenon (I worked on the catalogue ingestion systems for Last.fm a few years ago, so I have a rusty and likely out of date but still above-average familiarity with this topic)
On the surface, a music catalogue is based on a naive data model made up of Tracks, Artists, and Albums. Identifiers are basically made up of names and titles, or some composite.
Reality is (a lot) more complex. A slightly less naive, but far from accurate, data model includes the concept of "recording". Take Lou Reed's "Coney Island Baby". It's one "track", but there are actually 9 recordings of it (see https://songstats.com/track/35oekh1n/coney-island-baby). These "recordings" have unique identifiers (International Standard Recording Code). How are they used? Well, take two of them: USRC10300143, GBLLT2105923. Same track, different country (US, GB). E.g. the track was published in two different issues of the same album in different countries. And the licensing for each of those may be completely different (e.g. rights in the US may belong to $companyA and rights in the GB for $companyB). Or $companyC might come and buy rights for distribution in the GB, and perhaps decide to take down the GB recording.
For the purposes of your playlist, you want to listen to Lou's song, and don't care if you're listening to USRC10300143 or GBLLT2105923. But labels do, and Spotify (or any streaming service) has to care and report listens / downloads / etc. on the recording, not the track.
But.. when you add a track to a playlist you're actually adding a _recording_. So there are a lot of not-immediately-evident reasons why you may end up losing access to $track because someone took down $recording with a foreign key to $track. And when you go and search for $track, you're annoyed to find exactly the same track playable.
Why doesn't Spotify doesn't keep your playlists healthy? The basic reason this may be simply that the volume of metadata changes into music catalogues is surprisingly large, so not a trivial problem at Spotify scale.
A further reason is that it's actually not trivial. What are acceptable alternatives? Well.. replacing Bobby Womack's "Across 110th street" with Bobby Womack's "Accross 110th street - Original" is probably fine, but using "Accross 110th street - Drum and Bass mix" as fallback may not make every listener happy.