As explained at IGMP Snooping and DSA - #4 by LGA1150 , IGMP-snooping now works with DSA on MT7530 aka Mi Router 4G's Switch.
The question remains: why is the discovery of Alexa devices now broken?
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.