Tvheadend

TVHeadend version: 4.3-1915~gfebcf9818
Running in Docker container (image used: linuxserver/tvheadend)
Hardware: Intel NUC (Core-i5-based)

I'm having trouble getting the audio to work when casting IPTV streams to any of the devices I have (2 x 3rd-gen Chromecasts and 1 x Nest Hub).

On my TVHeadend server I have a mixture of off-air DVB-S channels from Astra 28.2E and a number of IPTV channels from the Netherlands and Germany, derived from the lists available at https://github.com/Free-IPTV/Countries. These all work without serious issue; I can use VLC to play anything from TVHeadend and it will do so without issue on both the Windows-based PCs I have and my Android phone.

The issue comes when I try to use the casting support built into TVHClient. When using the internal player on the app itself everything works perfectly, but when I cast to any device I will only hear audio when casting an off-air DVB-S channel (for reference, I have done the bulk of my testing with BBC1 HD and BBC1 SD, and both work without issue, but any off-air channel seems to work); when attempting to cast a channel derived from an IPTV stream, only the video will render, with the audio being conspicuous by its absence.

I have configured a casting profile within TVHeadend using the settings shown in the attached pictures, and have configured TVHClient to use this profile when casting.

What am I doing wrong here?