Thanks for your reply.
The MT mDNS proxy covers my AirPrint, Airplay, Sony TV and Roku devices.
It’s strange. With mDNs proxy turned off, my Apple Music app still shows the two Rokus. But clicking on them doesn’t make a connection. I would have thought showing up meant some protocol that can cross VLANs, but maybe a firewall issue that doesn’t allow traffic to pass in whatever direction it needs to pass to actually connect?