Is there an existing issue for this?
Current behavior
When someone mentions a user there is not mail notification if you use the autofill of the username.
Look here:

Only the second message got a mail notification
Steps to reproduce
Scenario 1:
- Write @username and autocomplete it so that the username is blue.
- Send the message
- No E-Mail for the User
Scenario 2:
- Write @username and do not let it autocomplete so the username will not be blue.
- Send the message
- E-Mail arrives
Browser
Google Chrome
Version
Self-hosted