A case of spec ambiguity, maybe? I can't find a mention in WebFinger or ActivityPub specs of usernames being case insensitive. Are @osma and @osma referring to the same actor? Is that up to implementation? How does a remote server determine which is correct?
Edit: I wrote above (at)Osma@mas.to and (at)osma@mas.to, but some part of the stack converted both to a lowercase mention during posting. I don't know which part, and what specs describes that.