This happens when forwarding a HTML formatted mail in Outlook (HTML is the default) and you do not change it before inserting a recipient.

To reproduce:

• click "Forward" on an encrypted mail (regardless if opened in a separate window or only viewed in preview)
  
• enter a recipient -> the header information of the forwarded mail disappears
  

This does not occur if you add something in the forwarded mail before adding a recipient.

The issue was first reported with VSD 3.3.0 and Office 2016 but could be seen in VSD 3.2.2 with Office 2016, too, as well as VSD 3.3.0, 3.2.0 and 3.1.26 with Office 365.

Additionally an inline picture in the (to be forwarded) mail is no longer displayed (not even as attachment) after sending or cancelling the forwarding. But it will appear again if you select another mail and switch back. I suspect that is T7269: Attachments vanish from forward encrypted message or at least related.