Why does he stream in 720p? I watch them on a 4k 50" display and they look poor on it compared to when Matt, Sam and Nat stream.
Very strange indeed. Might be a problem with the settings. I know that, because they use the same Train Simulator account in all cases, there should be no issue with resolution settings being available on the user end - unless Twitch is doing something...sinister...
dovetailgames isn't affiliated or partnered so they won't get guaranteed resolution choices (known as transcoding), I think they get randomly assigned during off-peak times. Partnered streamers have continuous transcoding options - Affiliate program streamers do not have continuous TC but have a much higher probability of transcoding options than non affiliates. Nothing sinister about it, that's just one of the drip down perks of partnership over affiliates over non associated streamers.
This quality issue could be down to the capture software/hardware. I recently changed my capture software and it dropped the quality to 720p for streaming. I had to manually go in and change the stream settings in the software to 1080p once I'd realised my first 2 streams with the new software were only at 720p.