BCC missing from emails - External contacts only

Hi. I was wondering if anyone has recently came across the BCC field is missing from sent emails when its external contacts?

Have tried many different options from Google suggestions along with trying to add it in via Developer mode etc and can not get the BCC to appear at all. This is happening for myself and many other users within the organisation. Seems to have happened randomly recently and unable to get to the bottom of it.

Unable to view it within Outlook or OWA. When sending emails to internal users via BCC they appear without issue, happening only with external. 

Thanks