On my Mi Router 4a Gigabit Edition RC4 broke the discoverability of Amazon Echo Devices from Spotify, so I can no longer cast music from spotify to my Amazon Echo-speaeker.
The same problem applies when trying to cast to a Sonos speaker.
It looks like either multicast forwarding, the conversion from multicast from LAN to WLAN (multicast to unicast) is broken or something like SSDP is being blocked.
[Problem does not occur on TL-WPA8630Pv1 which is QCA-based running 21.02.0RC4, so the problems seems to be MT76xx related, Maybe it is due to the lack of igmp-snopping support in hardware of the MT7530-switch? See (1) and (2)]
UPDATE:
The problem was related to a broken umdns-daemon in connection with seccomp.
I discovered the root cause when my dawn-daemon couldn't see other dawn-nodes on the same network.
Errormessage was: daemon.err dawn[x]: Failed to look up test object for umdns
The workaround listed on https://bugs.openwrt.org/index.php?do=details&task_id=3355 works:
cd /etc/seccomp
mv umdns.json umdns1.json
sync
/etc/init.d/umdns restart
After applying the fix to my Mi Router 4a Gigabit Edition and rebooting the router, my Echo-speakers were visible in Spotify again and the Sonos-app found the Sonos-speakers again.