Another one bites the dust ;).
My problem was strict vs loose routing done by loose_route function.
In order to loop through B2BUA I have defined the loopback interface's IP as a local alias for simplicity in checking myself in routing script. When doing loose_route, the route address was for some reason not matching my server and due to strict routing the RURI was rewritten with the weird address in the route header.
Hope my report helps someone in the future.
DanB