Not sure at what point this started happening, but I just noticed that every tab I have open is automatically bookmarked in a folder tree (under Simple Tab Groups, with sub folders for each group I have), and closing a tab doesn't delete the bookmark. I unchecked the option "Allow access to bookmarks" and deleted the created bookmarks.

With that being said, I love the add-on and appreciate the QoL improvements it brings to my internet. Can't imagine not having it at this point.