I can keep it simple.
Do you see the traffic in your logs? If not: arp issue can be verified with tcpdump, you will see that firewall does not reply with arp to the external IP.
Do you see traffic in logs, check if the NAT rule is hit. If not check NAT rule.
If this all above is not the case then make tcpdump capture on interface on where real IP is connected, if you see traffic going out but not back, it is server issue or network behind fw.
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