i am using an XG230.
To recreate: Create an outbound LAN-WAN rule, the rule will auto-group into one of the default groups.
Generate outbound traffic and review the packet capture looking at the NAT and Firewall Rule IDs.
It will always show NAT Rule #0.
I checked the same with Inbound traffic and a NAT Rule does show for DNAT inbound WAN-LAN rules.
Matt