Non-founder administrator is unable to select a "category" as a parent on creating/editing a forum in ACP.
The problem is here https://github.com/phpbb/phpbb/blob/master/phpBB/includes/functions_admin.php#L119

To get "cat" selectable in the parent forums list, admin has to have m_approve, f_noapprove (or f_post if $only_acl_post flag is set) permissions for it, which is not the case for "categories" as they aren't postable.
This is not the issue for founder as all permissions are forced to true for that user type.