I'm not sure how long this has been happening, but I just noticed it today. Apparently, this is not appearing to my other friends who also use DNDbeyond, only me. I'm confused if this is a concern, but they suggested I bring it up on the forums.

This happens when I'm on the DNDbeyond landing page, then hover over my username in the top right and select "Characters". No matter the name displayed, my characters are still present like usual. I'm on PC, so I'm not sure if this happens on mobile. 

I saw another person report the same problem (different names involved). The characters were still his though.

This has happened to me as well. I was starting to wonder if someone was messing with my account.