Plex has/had a long standing problem that playlists are bounded in size, including smart playlists based on SQL matches in the DB. We're talking 5+ years marked down as "yea when we can"
Google had a perfectly cromulent music system which included uplift of your own MP3 and a very good UI. They canned it to combine into youtube. From memory it did not have this problem. Classic enshittification.
