Same here.
Installed 102.6 and did a factory reset (WPS reset or Administration - Restore). Manually config all the settings I use and UPnP is enabled. Xbox Series X shows NAT Type: Moderate.
View attachment 69414
Reinstalled 102.5, factory reset (WPS reset or Administration - Restore) and Manually config all the settings I use with UPnP enabled and my Xbox Series X shows NAT Type: Open.
View attachment 69415
Sorry for going a little off topic here:
It doesn't look like a Asuswrt-Merlin issue because it happens with the factory firmware on the ASUS GT-AX6000 too.
Also done with factory reset (WPS reset or Administration - Restore) and manually config all the settings with UPnP enabled.
ASUS GT-AX6000 Firmware version 3.0.0.6.102_34825, UPNP enabled works.
ASUS GT-AX6000 Firmware version 3.0.0.6.102_34860 and 3.0.0.6.102_34876, UPnP enabled doesn't work correctly on the Xbox.