This involves 3 employees and 1 external person.

The first employee sent an email to the external person and BC to another

The BC employee in turn forwarded this email as an attachment to a 3rd

When this 3rd employee opens the attached email, it shows the original
external person in the TO field, and it shows himself as a BC recipient. So
the help desk call -- "why didn't I get this email when it was BC'ed to

Other emails like this that do not involve BC seem to display like you would
expect -- just showing the original TO and CC. Is it a glitch with BC or is
it designed to protect the original BC or just to show that they have now,
in effect, been BC'ed.

I'd have to guess it's a glitch.