This is currently intended behavior, because in the past, the category was never migrated when moving bugs between projects (it was just a string). For the time being, the feeling is that until we can generate a "proper" interface for determining bug category at the time of movement, it is better to keep the old category in place, and let an intelligent updater make the proper decision to change category appropriately, rather than make any (possibly false) assumptions about the new category. The fact that the category is still prefixed with the old project is meant to be an immediate clue to the user that it needs to be updated post-movement.

However, it would be beneficial to, at some point, devise and implement a simple method for reassigning bug category during movement. Patches / submissions are welcome, of course.