I noticed during a conversation with User ᴅᴀɴɪᴇʟッ under a post of mine that the last character
ッ does not appear in the autocompletion when I start my comment with
@ to ping them. Most probably this is because
ッ doesn't come from a "standard" Roman script? I believe the pings are going through, but the best person to confirm or deny this would be ᴅᴀɴɪᴇʟッ.
One can test this by picking any comment by ᴅᴀɴɪᴇʟッ from their activity page to reply to. (Of course, there is no need to actually post a comment to test this, so I'm not inviting spam to rain down on them. I hope.)
Can the username autocomplete please be made to pick up all the characters?