To tack on the end.
All usernames are Lowercase.
Default Twitch will render a username with the first letter capitalised. If the user hasn’t set custom CaPitalISAtion on their name.
Over IRC you should always address a user by their lowercased name.
I anticipate that the web frontend auto complete uses the users selected capitalisation and ember handles this by lowercasing before send.
So in summary, on IRC you get given the name lowercased (and the CaPitalISAtion version is in IRCv3 tags) so just address a USER by their lowercase name for Whispers. Saves yourself some hassle if you programmatically use lowercases