It's normal. First, each ssid, you can consider it as interface like ethernet. The DAP support vlan, and all the ssid work in hybird mode, and the lan interface works in trunk mode. Normally the hybird mode interface can tag or untag vlan (different from access mode), but consider all the ssid will only connect to client instead of another L2/L3 switch, so all the frame sent to client should clean the vlan tag or else the client can't understand the frame. That's why dlink ungred the tag, no reason a radio interface should link to a swith, it's always the lan interface links to a switch, so you see the lan interface you can tag or untag.