Spotify will add songs that I search and listen to into my playlist. I didnt add them nor did I tap the more options icon, I just play the song. It will also occasionally do it within the same playlist, it will take songs that I listened to, and RE-add them.
The first image is of the song in the same playlist, and the second one is of song that I searched and spotify just added them for no reason.
in the playlist alreadysearched but I didn't add, spotify did