Dolby Atmos sound cutting out

I have a Denon AVR2400H and my Xbox one is connected to that via HDMI. I have set my Xbox to output Dolby Atmos for home theaters. When I have been watching the grand tour, the audio keeps cutting out, when I adjust the volume it comes back for a short time. It doesn’t do this with any other apps, such as Netflix, just plex. If I change the sound output back to just Dolby Digital it seems to be okay, but it does sound better with Atmos so I would like to get it working. Has anyone else had this issue?

The plex app doesn’t support atmos so most likely there is a problem with transcoding.