New Spotify user here and I've searched for an answer to this problem but can't find it.
When I play a playlist, one song plays from the playlist then it plays songs outside the playlist, specifically modern pop songs.
I'll try to explain exactly what's happening here. From the home page in the app, I click on the library button which takes me to the list of playlists I've followed. I click on the playlist and it comes up in the main window. Then it doesn't matter whether I click on the first song on the list or the green Shuffle Play button, one song will play from the playlist then modern pop songs will play. If I hit Shuffle Play, it will then play a song from the play list but right after that it goes back to modern pop songs. So after every song I want to hear, I have to hit Shuffle Play to skip the modern pop song that comes after it every time! Very annoying.
The playlist is Alternative 90s which has 75 songs, so I am sure none of these modern pop songs are in this list. I looked through it just to be sure, yep, all 90s Alternative songs.