Tag is being assigned at Post creation by default

Hi. I’m using the instructions at https://publishpress.com/blog/permissions/stop-users-creating-new-tags/ to limit tag creation for several user roles. When someone in a limited role creates a new Post we’re finding that one of the existing tags is being assigned immediately, even before the Post is saved or edited at all. There are several tags available on the site but the new Post always immediately carries only one of these, and the user must delete or change the assigned tag every time a new Post is created. What is likely happening here and how can I limit tag creation while keeping the tag field empty until/unless it’s needed?