I have been experiencing a similar issue with our Roku Ultra where the volume either goes down or up randomly. I am typically using YoutubeTV when we notice it, but I believe it has happened once or twice with a different channel.
When my wife and I experience it, she is often watching something on WeTV. If the output volume of what is airing is low (silence or near silence in a scene) when the problem occurs, it tends to cause the volume to go lower. If it is loud (action, yelling) the volume tends to go higher.
Our method for fixing it is to switch to a different YoutubeTV station or go back and reload the station. I wasn't sure if it was Roku or YoutubeTV (or a combination of both) that might be causing the problem. I haven't observed a similar issue when using YoutubeTV on a different device (tablet, PC).