Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
So I just ran into a similar issue on my Roku Ultra units w/ Google Pixel 3a devices (fully patched.) It seems that the issue started happening after a recent update (either to the ROKU or phones, I'm not sure which.) To resolve the issue, I did a 'force stop' on the Youtube app, and then cleared the cache on each phone. After I did this, the ROKU units both appeared in the 'Cast to' options from within Youtube app on the phones. Woot!
My ROKU Info: Roku Premier+ (Wireless | Static IP) - HDMI (1080P) Roku Premier+ (Wireless | Static IP) - HDMI (1080P) AT&T Internet (1024 Mbps / 1024 Mbps) Netflix; Hulu Plus; Amazon Prime; HBO Max