In a routing policy, if both an interface and next hop IP address are configured for an egress, the interface is prior to the next hop IP address. However, before an interface serves as the egress, the interface must be configured in not only the routing policy but also the next hop of the static route on the firewall. If an interface is configured as the egress in the routing policy while the next hop of the static route on the firewall is an IP address, this problem occurs. The egress configuration in the routing policy and the static route must be the same.
Suggestion: You are advised to configure next hop IP address rather than an interface for an egress, because the large quantity of ARP requests sent by an interface bring down the device performance.