It’s all likely A/B testing. My TV doesn’t have it. My iPhone 11 Pro Max logged into my YT account has 4K for HDR content. My iPad Pro 12.9 2020 has 4K for all content, SDR and HDR, but only if logged OUT of YT.
Nice find, went into incognito mode in the YouTube app on my xs max. Every video I played was running at 2160p😊
most likely a bug that YouTube needs to iron out.