It doesn't happen in youtube, but something I've noticed recently with Plex Web (and Plex IOS for that matter).

I used to be able to play video via plex web and using the cast media button I could cast the audio to my JBL link Music - a google chrome cast enabled speaker.  In Chrome/Edge, it does show an actual speaker for the icon rather than ea TV.

But have found recently that is I cast the audio, Plex and jellyfin  asks if I want to continue playback on the JBL link music, if I agree I start hearing the audio on the speaker, and I get a blank video screen on the browser.

Has anyone else noticed this?  Or do I just have a weird way of playing things.   

I run Plex web on my rdp only session, and rather than listen to the remote audio via headphones I was casting the audio to the speaker I have next to me.