When the permission for a specific forum (tested with a category) is set for multiple new user groups (e.g. administrators and registered users) simultaneously, the permission role that is selected for any of the groups is synchronized for all other groups. Also, the list of roles in all select fields is repeated as many times as there are groups to select roles for.
This ocurrs regardless of whether existing permissions on a forum are altered or new permissions are added.
Setting the permissions for only one group at a time works.
(thanks to phpBB.de User Juppi)
The arrows marked with 1 show the repetitions, the arrows 2 show the synchronization of all rows.