Multi User Picker remains with a z-index of 9000 after autocomplete.

Add a 'Multi User Picker' custom field, and a 'Date Time' field below it.
Now create an issue, and add a user using the autocomplete feature of the Multi User Picker.
Then try to set the 'Date Time' field below it using the popup date picker.
The user field will appear above the date picker.

The div surrounding the user field textarea gets a z-index of 9000 when the autocomplete appears, but the z-index is never removed/reset after - causing it to appear above the date picker.