When I receive email notifications from bugzilla, the X-Bugzilla-Comment header is either Public or Private, but there is really a third state, which is when the comment is empty. When there is no comment, it seems to be marked as Public. I would love to be able to filter my incoming and throw out things that are say only a change to the CC field when no comment is added.
This will be fixed in the next release. It has been tested internally but it cannot be tested yet on partner-bugzilla as email is disabled of course. Please reopen this post-migration of you do not see the header properly.