Interestingly, I made a new zimbra test machine; exact same VM & OS settings, and this time the GUI button works.
Just thought I'd toss that out there.

Ah!
When I set it via the command line with your commands, it worked (I must have been mistyping it before).
However, the GUI checkbox doesn't appear to work.
I set it back to true via the command...

I am evaluating Zimbra (I am quite familiar with its underlying technologies however) and I too have this problem of the X-Originating-IP not being able to be un-set.
However, not running my inbound...