Check to see if the playlist is 'enhanced'. A while ago, Spotify introduced this capability. When activated, it inserts suggested songs intothe playlist.

Huh that's pretty wild. do you have bluetooth devices connected, or within like 40 ft and could be auto-connecting, when this happens? That's one of the other things that I feel like could trigger it. Although going straight to random songs not in your library is somewhat unusual behavior even if it were this, I still think it's possible

I know I have friends with iphones that will start playing ancient itunes music they own when they autoconnect to my head unit lol

i get that shitty auto-play recommended tracks after a playlist bullshit because i use a downpatched adblocker-friendly version of the client and the button to change the setting doesn't exist. i could switch back to the browser version but i don't want to take the quality hit.

You might possibly be hacked from what I'm reading. Spotify can let you control music on one device from another.

I noticed that it can take a while to switch from mobile to desktop, but not the other way around, so maybe someone has access to your account, tries to play it on their device but it starts on your mobile first. Check in your account options for anything about connected devices and log ins

Does it occur on different platforms as well?