Selected (and deleted) users/subgroups are not displayed properly in drop-down when editing a group

STEPS TO REPRODUCE:

1. Login as Admin
2. Create 3 users (e.g. U1, U2 and U3)
3. Go to Administer Wiki > Users & Rights > Groups
4. Create and edit a new group (e.g. G1)
5. Click on 'USERS TO ADD' field and select an user from the drop-down
6. Delete the selection
7. Click again on 'USERS TO ADD' field to see the remaining users available for selection

EXPECTED RESULTS

When an user is selected and then that selection is deleted, the respective user is listed again in available selections drop-down.

ACTUAL RESULTS:

The user is not displayed anymore in the list of available selections drop-down (even if clicked many times in the selection field to activate the drop-down). However, the users are displayed again if they are searched manually by typing or the modal box is closed and then opened again.
The same thing happens with subgroups.