Maybe this is a bug? I have several users whose first name starts with the letter 'C'. Let's call one Chad Testerson.
In creating a JIRA bug, if I type @c - I would expect a list of names starting with the letter 'C' to auto-pop in a dropdown and let me select the user. It does this for any other letter.
However, if I type @test - Chad's name will show up in the list. THEN, if I type @c - the popup comes up and Chad is listed.
The same goes for any other user that starts with the letter 'C' - they won't show up in the dropdown until I @lastname them. Then they show up in the dropdown when I try it with @c
If I refresh the page and try again, it goes back to not working unless I repeat the hack of using their lastname.
I also have a user whose last name starts with 'C' - and the reverse is true for him. I have to type in @firstname in order for him to show up in the dropdown. Once I've done that, I can @lastname and he'll show up.