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So, was last night I discovered that even though Stadia was saying I had an 'EXCELLENT' connection (green icon in Connection), 4K was saying 'unsupported by network'. Speedtest.net was showing Ping 25ms D/L 141Mbps U/L around 40Mbps. All other platforms streaming 4K no problem. YouTube 60fps 4k HDR sample videos no judder or stuttering. This same issue persisted on two different Chromecast Ultras and my laptop. I even moved my Vodafone SIM to a phone and hot-spotted off that - again, similar speed test results, Stadia showing 'Excellent' connection and green icon, but 4K 'Unsupported by network'.
In chat with Google support assistant he confirmed 3 times that Stadia should run in 4K on 4G and 5G networks if the connection is fast enough.
In chat with Vodafone support they confirmed that, quote unquote exactly, "... Vodafone are not throttling Stadia and are not preventing it from running in 4K".
I showed a direct screenshot of 5hat quote to Google, and they are going to come back to me in the next 24hrs.