hi @fortimaster ,
Please be informed, Policy route will override normal routing table.
I will give you the general idea.
192.168.1.0/24 GW A
192.168.2.0/24 GW B
192.168.3.0/24 GW C
Traffic will route accordingly to the GW A,B or C.
192.168.1.0/24(destination) GW D
Means, forced this subnet to go to GW D only.
If traffic arrive to Fortigate on GW A, Fortigate respond using GW D.
Coming GW A, Respond back GW D.
This create asymmetric routing issue and traffic will not work.
You can specify "source" and "destination" to make it more strict routing.
However, policy route mostly use because of network design issue.
if the network design is correct, policy route should be avoided.