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Why are some of my favorite songs disappearing from my playlists and why are some artist albums incomplete?
My best guess is that the artists whose songs are disappeared are undergoing some licensing matter or have put in other decisions.
The missing songs are not erased from the database, they are simply greyed out.
If you're on the desktop player, you can enable showing greyed out tracks, it's in the Preferences. By default it's disabled.
If you have more questions, feel free to ask 🙂