Recently Played keeps a specific amount of recently played material and if you're listening to a wide variety of music, it will remove the oldest albums/artists/playlists in the selection.
If also the very recently played artists disappear from the list, could you try another device with your account to see if the issue persists? I also suggest taking a screenshot of the Recently Played now, and again when you see some artists missing. 🙂
Let me know how you get on!
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