Can confirm that it also doesn’t work on my end. Even with my phone’s native screen screen record capability.
As outlined here, apps can prevent audio from being captured. Not sure why Twitch would allow video recording but not the audio, though.
I’ve created a support case with them, we’ll see what they respond.
You could also do the same on your end, it could help prioritise the issue. Twitch Help Portal
New update, we’ll assume that Twitch is affected by the WebView bug mentioned here.
Right now, you could try the unstable canary build or wait for the update to the stable channel.