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.
To my knowledge there are no specifications for this. It's left completely to the implementation. Some projects have case-sensitive usernames and some do not. Some provide case-insensitive search and some do not. As a general rule, you'll often avoid issues if you use whatever form was used by the source of the information that you got it from.
@mikedev
Yep. I only ran into this when my server received messages with mixed conventions... Didn't have a unique lowercase index on the actors, so ended up having two records for the "same" one -- for informal understandings of sameness.