'Private' checkbox is available for Administrators
I am not sure if this is a bug or a feature since I haven't found a description of this field on the wiki. But it seems to be a bug.
I deactivated in the permissions to use the 'Private' checkbox for new issues for all available roles. If I create a new issue as Administrator then I have the 'Private' checkbox available. If I select it, other Administrators can still see the issue. Is this how it should work?