You do not need policy-based routing for static routes.
You may have introduced some kind of asymmetric routing in the process, especially if that other network is reached via an IP on the same network where the client is.
With the gateway as default, the firewall will send back an ICMP redirect and the client will talk directly to the other router. With PBR, the firewall will try to forward it there on its own.
You probably also want to go to System > Advanced on the Firewall/NAT tab and check the box to bypass firewall rules for directly connected networks.