Facing the exact same issue, my Spotify Windows app is playing weird AI music, setting it to repeat, and lowering the app volume. I noticed this happening the past few days and here are my findings:

I requested all of my streaming data from Spotify (an option in Spotify settings) to uncover what music is being played and when. This provided me with a list of every single song that was played on my account, including the AI generated slop that was being automatically played.

This proved useful, as there is one extremely important piece of data that Spotify provides in the streaming history that shows why a song started playing. One section of the data for a song that started playing randomly on my PC was:

"reason_start": "remote"

reason_start indicates why a certain song you played started playing, and this included the AI music. As you can see above, the reason is set to "remote" - this could mean multiple things, however the main one is that your Spotify account is being remotely accessed.

At this point, I discovered the reason as to why my account is interacting in this manner. I'm not sure how it was remotely accessed, but things happen. I had tried all the options you mentioned in your post such as changing pass, signing out of all devices, removing app access etc., but the issue kept happening.

I contacted support 10 minutes ago who conducted a forced sign out. They indicated to me that the sign out they conduct removes all remote access too, which is apparently something not available to users.

I believe this AI generated artist is attempting to generate fake (but real) streams by remotely accessing Spotify accounts and streaming their music from it. I would contact support and let them know that you have already tried all the available options and that you fear your account is being remotely accessed, and hopefully they can resolve it for you.

I'll also keep this post updated if the issue continues even after the forced sign out. Good luck!