In case that was unclear: When xyz member leaves or is removed from a group, all their messages in that group lose their display name as being by xyz, and instead show three dots where the display name used to be. But if xyz is then banned, their display name comes back below all their messages.
It does not make sense for banned members' messages to show display names when left/removed members' messages do not.
And if, in fact, a group moderator wanted to exploit that loophole, they could simply ban any past member to make the display names beneath their messages come back. (I would certainly never do that. Not moi! :)
Messages are the sole opinion of the author, especially the fishy ones.