@Virgo 

Relevant network topology is:

ER605 v2 (2.1.2 firmware)

TL-SG2218         TL-SG2210MP

Printer                  EAP670 x2, EAP615 Wall

                             Clients

The described behavior of AirPrint failing occurs when the printer and client devices are on the same VLAN, so I'm not sure an mDNS rule should be needed. Will a multicast packet not transmit through the gateway on the same VLAN? Using the Flame iOS app to view broadcast services, I see the same set of advertised services whether the printer is wired or wireless. It's making the connection that is the issue. 

I have also set up mDNS gateway rules for the default Printer and Scanner bonjour services to try to make this printer visible to other VLANs with the same issue of not working when the printer is wired. But I also generally don't think mDNS is working correctly at all since none of my rules result in services being broadcast across VLANs when viewing advertised services in the Flame app (eg androidtvremote2 or spotify-connect). 

  @Virgo 

hmm. I'm not  IPv6 for anything and it's turned off for the VLAN of the printer. Could that still be affecting it?