This info gets cached in the Mylyn client. To clean this cache up and re-pull the info from the server, select the task that is showing the old attribute and delete it from the Mylyn task list. If this task is part of a query, you will notice that it is still available (just the cache was deleted). Regardless, next time you access this query it will show the correct attributes.
For more information about working with Mylyn see: Working with Mylyn task repositories