Source IP address of Web Proxy can’t be configurated in Winbox and WebFig. but it’s OK in terminal.
Have you reported this to support?
If so, could you please also add it here: http://forum.mikrotik.com/t/known-issues-and-bugs-a-list/71473/1
If not, please report to support@mikrotik.com, or let me know I will put out a ticket.
Also, its worth noting that if you configure something in Winbox/Webfix, it will still be displayed, but what is really configured in console can be different.
So you can have a different config in Winbox/Webfix and in console, which is a major inconsistency. I would guess whats in the console is correct tho.