AirPlay seems to work for everything except one App

I am using an iPhone 11Max Pro with the latest iOS and a 4th Gen AppleTV with the latest TVOS.

When I mirror my iPhone to the AppleTV I can see my phone screen and hear sounds and see video playing. When I click a video and then orient my phone horizontally I get everything working exactly how I'd expect.

However, with the GCN (Global Cycling Network) app as soon as I try and click on the video content the AirPlay connection seems to drop and the AppleTV screen returns to the app menu home screen. If I approach the app whilst NOT mirroring and start the video on the app and click on the AirPlay symbol in the video in the app, the symbol goes red and nothing happens.

The evidence seems to point to the app itself although I have friends who say they have it working although the current climate makes getting together for me to see it for myself problematic.