For a router, there is the Routing Information Base (RIB) and the Forwarding Information Base (FIB). The difference between these two is that while the RIB contains all possible routes to various destinations, even if, there are more than one to a specific destination, the FIB contains only the best route to each destination.
https://docs.paloaltonetworks.com/pan-os/10-2/pan-os-networking-admin/virtual-routers/virtual- router-overview

To work around this issue, one possible troubleshooting command is set deviceconfig setting session tcp-reject-non-syn no which disables TCP reject non-SYN temporarily (until reboot)4.
This command allows non-SYN first packet through without dropping it. The flow_tcp_non_syn_drop counter increases when the firewall receives packets with the ACK flag set, but not the SYN flag, which indicates asymmetric traffic flow. The tcp-reject-non-syn option enables or disables the firewall to drop non-SYN TCP packets. In this case, disabling the tcp- reject-non-syn option using the "set deviceconfig setting session tcp-reject-non-syn no" command can help work around the issue. This allows the firewall to accept non-SYN packets and create a session for the existing flow.