That would be one cause. Another could be character transfers. The forums save your activity based on a specific character and if you transfer that character, you get a new forum profile for the new character. The old activity is still there and visible.
So if you’re doing multiple transfers and end up back on the original realm you could have this sort of ‘duplicate’ profile.
The avatar has always updated with whatever gear I last logged out with on TBC Classic.
Haven’t changed characters in the character selection either.
No transfers have occured.
There are people (characters) with thousands of posts some even tens of thousands, of which would had to have been from Vanilla Classic and TBC Classic combined: and their post history reflects that.
Making a bug report in the website bug report forum would be an option, yes. However I do want to make sure there are some expectations set - posting a bug report does not guarantee a response from the website team. It also doesn’t mean they’ll be able to do what you’re requesting. Sometimes fixes to bugs are not retroactive, so if they do find a bug in the forums and they are able to fix it that doesn’t mean they’ll be able to merge the two separate profiles that exist.