Thank you @pminarik. This is definitely causing the mess in our design.
I have raised a TAC case with Fortinet and the engineer has suggested the following:
a) Redesign the network and not use the 0.0.0.0 IP.
b) Downgrade the FW.
There seems to be no other option. I did read the documentation and is there anything else we can do to get this working. We have about 10 sites which are using the exact same design. Any other way of getting this fixed?
Appreciate all helps - thanks.