Hi, I am experiencing the very same issue. I have Archer C6 v2 device, bridges are set up with IGMP snooping configuration. Once the interface (bridge) is fixed for the SSID, then the mDNS is working fine. It doesn´t matter if the auth is WPA2-PSK or WPA2-Enterprise. I can stream to AirPlay devices etc. But once I´ve changed the config for the very same SSID to the dynamic vlan (same config like @meresgr posted), then the mDNS is not working and I cannot find any AirPlay targets nor other services using mDNS or IGMP. I can confirm, that after switching to dynamic vlan, the same client device ends up in the same bridge. The only difference is, that the bridge is not assigned statically in the SSID config, but rather dynamically from the RADIUS server.
I´ve investigated the traffic on the bridge via tcpdump and it looks the same in both configurations. Even with the dynamic vlan, I can see the broadcast requests from the wireless client and resposes from the targets on the bridge, but probably they are not propagated to the dynamically associated wlans.
I can see the same behaviour on 2.4 GHz, 5 GHz and also on v19 and v20 rc3 versions of OpenWrt. Can anybody help us or point us somewhere to raise this issue?