I have three Roku on my local network: a 4660RW (Ultra), a 3600X (Stick), and a 3100X (2 XS). Two are wired and one is wireless. All three are running the latest Roku software, 8.1.0, and all three were restarted today. None of them are visible under Chromecast on the audio settings tab. I also have a Chromecast Ultra and it is visible to Roon Server, enabled, and working from Roon. All other devices on my local network (MacOS, iOS, Windows) can see and cast to the Roku boxes.
Roon server is running 1.5 build 339 on a CentOS 7 server. My network consists of two Netgear GS switches (a GS716T and a GS724T) and two UniFi wireless APs.
The router is a Netgear WNDR4300 running dd-wrt v24sp2, but that should not be relevant to the issue…