Based on the configuration of the WAN1 port, PPPoE with VLAN tag, shouldn’t the port be marked as tagged?
No, because the tagged or untagged icon displays the status of the VLAN you have selected in the VLAN dropdown, which in your screenshot is “1”. If you were to select “6” then it should display tagged.
VLAN 6 is not a “local”, “inside” VLAN. It’s a VLAN provided by the ISP. There should only be one VLAN on that port. All the others have no business being on the WAN port. 