User properties is not removed when user removed from external user repository

Here are the steps to reproduce the issue:

1. Install a new Confluence instance.
2. Add a new user directory with LDAP (or Active directory).
3. Create a new user "testuser" on LDAP.
4. Synchronized the user to Confluence.
5. Login with the new user and change the user setting like "Notify on my actions" for example.
6. Logout and remove the user from the LDAP then perform the synchronization on Confluence again to remove the user.
7. Look into the table "os_propertyentry" of Confluence, the related user properties are still there.

It can be reproduced on Confluence 5.2.3 as well.

The issue is not affecting any functionality of Confluence, but when upgrading from the version 5.1 to 5.2.3, it will show up the warning message in the log file:

2013-08-22 12:04:09,192 WARN [main] [atlassian.confluence.upgrade.UpgradeTask] matchUserKeysToUserProperties The following 1 usernames could not be matched for user property migration and will be ignored: [testuser]

Which cause a bit confusion.