Thanks for reaching out to the Community and welcome!
Hmm that's odd. Songs can't be removed from a playlist on their own, so maybe they've become unavailable in Spotify (if that's the case, they should stay in the playlist but greyed out, though), or maybe they're being removed by someone else. To secure your account, we suggest following the steps in this article.
Hope this helps. If you have any questions or need anything else, the Community is here for you.
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