Same for me, on 1310.4 firmware and Samsung 55" Q6FN model. YouTube video with a 1080p resolution and enabled "stats for nerds". At least in my case on the "connection speed" it shows 50+Mbps and on the "buffer health" it shows +60 seconds. But when I change it to 4k, everything stops, connection speed goes to 0, buffer health goes to 0, network activity goes to 0. The last Samsung firmware broke the YouTube app, I have zero issues with Netflix, Apple TV, Amazon Prime Video.
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