Tested in JIRA 5.2.7
Affected system fields: Assignee and Reporter

When using the usernames to search under the 'Suggested Users' list, user will not able to find users by typing the username, but if they type in the Full Name, the user appears.

As an example, I have a user with:

• • Username: checking
  • Full Name: name
• We can see that the user is under the suggested users list:
  
• When I type 'name' the user appears. Shown as:
  
• When I type 'checking' it returns no match. (Because this user is under the suggested list)
  

However, this seems to be improved inside JIRA 6.0.x. In JIRA 6, you can search the user by both username or Full Name, even the user is under the suggested list.