As I cannot log in tho the main site due to some unknown reason, I post the bug here in the hope that someone reads it and posts it to the issue sector on the main site.
THE BUG: Milestones cannot be set confidential in Public mode.
==> This site is set public but all issues are confidential,
so the Milestones should also be confidential.
But as you can see in my example they are visible.