Hi Guessed,
The circle with the CT is the users initials. That screen shot is from a MS Teams screen share session.

I can't post a document because even a new fresh document acts this way. As noted, if a different user logs into the same computer, the problem goes away. I'm almost certain this has something to do with the user's account or preferences but I am not sure exactly what.

I don't know how else to explain the problem so I posted a video here: Word Styles a Problem - YouTube

As you can see, some of the styles take and some do not.