How do I unpair my windows license from my old PC

I recently built a new PC, and used my windows home license from my old custom built PC to activate windows. This happened automatically as I logged into my microsoft account.

Now I want to sell my old pc, so I've done a full clean install of windows and fully cleared the drives. I also removed the old pc from my list of devices on my microsoft account. After reinstalling windows on the old pc, I bypassed the login offline to see if everything was successfully deleted without having to log into my microsoft account. I also checked my key activation. I then plugged in my ethernet cable, and almost instantly windows notified that it was activated. 

Despite my license already being active on my new pc, and being removed from my devices on my account, it still automatically pairs with my old pc once it connects to the internet.

I've ofcourse already tried slmgr /upk and slmgr /cpky, but this didn't fix the issue, since once I press the "resolve issue" button next to the missing license, it just automatically pairs with my personal license. Microsoft most likely just has a database with pc and license pairs, to conveniently auto-connect product keys to pcs. my old pc needs to be removed from this, and it's ridiculous that there is no easy way to do this, if there is a way at all.

I'm afraid that when I sell my old pc, the new user will automatically pair with my license, and (possibly?) throw my new pc off my own license. Does anyone know how to resolve this issue? I've already had 2 chats with microsoft support, and they had no idea what to do. I could ofcourse throw a 3rd party OEM license on it, but this doesn't seem like the intended behavior of product licenses.