Air play for videos

When I select my HomePod mini to play the audio of a music video (Apple Music) that I am watching on my iPad it transfer the audio to the HomePod Mini but stops the video. It shows a still only. I have had occasions where it didn’t do that so I am confused about how to do this. Airplay will not allow me to select both the iPad (source) and HomePod Mini at the same time. I have a 2020 iPad Pro running iPadOS 14.3.