Upstream report: https://secure.phabricator.com/T9479
Typing @C933103 does not link to the users Phabricator-profile (or ping/subscribe that user) even though there is a user with that name: https://phabricator.wikimedia.org/p/C933103/
This is due to there is only one letter in the username. Trying to add another letter anywhere in the name will cause the "username-tag" in the preview (below) appear red (no such user exists).