static natting

The PBR seems to be configured for Internal to external traffic. This does not assure that when the traffic is initiated from outside, the FortiGate will allow the traffic as RPF ( Reverse Path Forwarding) check will be done initially by the FortiGate and if FortiGate does not find the route of the source associated with the incoming interface, then FortiGate will drop the packets from that source

To see if the FortiGate is blocking the connection due to RPF check or not, you can run the below commands:

diag debug flow filter addr <source IP>

diag debug flow show iprope enable

diag debug flow show function-name enable

diag debug console timestamp enable

diag debug flow trace start 1000

diag debug enable

Then initiate the traffic and see if you see any rpf check failed messages or not. Also, it will show how FortiGate is processing the packets.

Also, you can take the packet captures in the first place to see if the traffic is arriving on the FortiGate or not

diag sniffer packet any "host <source ip>" 4 0 l

If RPF check is failing then you can disable RPF check on the incoming interface

config system interface
edit <interface>
set src-check disable
end

https://community.fortinet.com/t5/FortiGate/Technical-Tip-How-to-disable-Reverse-Path-Forwarding-RPF-per/ta-p/193338?externalID=FD51279