When adding official bug tags "Save" button stays disabled

On https://bugs.edge.launchpad.net/rosetta/+manage-official-tags, if I:
 * Use "Add a new official tag" input box to enter a tag that is already in the list box of "Other tags"
 * Then click "Add" button

New tag gets added to the list of official tags, but "Save" button stays disabled, so I can't really add it.

It works if the tag I am adding is not in the "other tags" list or if I use the arrows to move it from one list ("other tags") to another ("official tags"): save button gets enabled as expected.