User mapping goes wrong when changing user's Username in JIRA

When the user base is managed by JIRA and the Username of a user is changed, the User Mapping of this user gets blank in the column User. When editing the User Mapping, it does not seem it is correctly mapped, so maybe User Mappings should be reviewed when the user base is re-synchronized.

Steps to reproduce:

1. Have a repository in FishEye with at least one commiter to test.
2. Create a user in JIRA (e.g. Username = "grefosco"). JIRA used is in version 6.2.5.
3. Configure FishEye Authentication to use JIRA (create JIRA User Server, connect FishEye to it, synchronize groups, etc). The only setting I changed from default was When synchronisation happens, checking the option Copy new users and changes to existing users.
4. In FishEye, go to User Mappings and create one for the JIRA user grefosco and commiter grefosco.
5. In JIRA, edit the user, changing its Username to "gustavorefosco".
6. In FishEye, go to Authentication settings and run a Re-Sync. Ensure the new Username is showing up in FishEye.
7. Go to User Mappings. The column User is blank.