Adding a Campaign - Removes Associated Email Address?

Hi fellow Suite’ers!

I would like to first say hello, I am new to the forums :slight_smile:

I just wanted to run by a question to the community who may be able to explain what is going on when I add a campaign to a lead / account / opportunity on Suite 6.5.

Version 7.1.5
Sugar Version 6.5.20 (Build 1001)

When I add the campaign to the object respectively, the associated email address in the object, IE Lead email address, is removed. It cannot be re-added.
When I go to remove the campaign again from the object, I am still unable to save the email address.

This is very frustrating and counter productive, as we can get many new leads a day then assigned to a campaign only for the email address to vanish?

Is this a bug, or is this by design, just my lack of knowledge is at faulty here?
Any advise or clarity on the situation give would be much appreciated.