YouTube TV automatically resumes the last channel watched after reopening the app
That would be a YouTube TV setting. You may need to dig through them to see if there is something that may apply.
Possible this is related to the "Instant Resume" feature that is supported by the Ultra 4850 and implmented by YoutubeTV on this device. There are a few related threads in the forum discussing this.
Try pressing the Back button to exit YoutubeTV rather than the Home button to close YoutubeTV. (by backing out of YouTubeTV, this will interrupt the call for Instant Resume.
If implemented correctly, the Live content is supposed to start playing 1 second after loading when using Instant Resume so not sure why yours is paused, unless still a work in progress by Youtube TV.
https://developer.roku.com/docs/developer-program/media-playback/instant-resume.md
