Maps will not remove pin

Are these locations circles with white borders? There are two different places to save locations. You can save them to "My Favourites" and you can save them to "My Places". I just check and it seems I had some locations in "My Places" that I haven't been to for years. You can find "My Places" under "My Guides" in the sidebar (on a Mac).

Interesting ... since you see the same on your Mac, this pin's location is in your iCloud data, so even if you didn't transfer data to your new Mac the pin comes from iCloud. You say that you signed out of your Apple ID (good idea) but did you do that on all devices such that they were all signed out at the same time? Whatever this is, it would have to be coming from your iCloud data to be seen on multiple devices, so signing out completely (and maybe restarting all devices) might clear it.

Do you recall how you set this pin if it was for a specific contact? Just curious if you used Siri or some other service like Reminders (not sure if you can even set a location that way).

Assuming that you can tap on the pin to bring up the details card for the marked location, is there no red "Remove" button? (should be right above "Share") If you see Remove, is it not selectable, or does it just not do anything when you tap it?