My WAN 1 is down so using WAN 2 to test but that should not affect outcome.
Have configured port forwarding on matching both links, LAN server confirmed is active.
Done a packet capture on both sides, it appears that MX has dropped the SYN/ACK from the server and therefore 3-way handshake not completed.
Removed all firewall rules (actually the rules are not blocking anyway), still the same. Any idea why MX is dropping the SYN/ACK only for port forwarding traffic?
Other outbound traffic is fine.