I believe if your watching thro app they have taken the feature out you have to go to the 3 dots at top right then click quality and scroll to 1440 4k if they have uploaded for 4k viewing hope this helps
Because, simply put.
It's not a 4k screen.
Maximum YouTube will display for S10 series is 1440p60 HDR.
I believe only the Sony phones can currently play the 2160p60 HDR streams
Because the camera can record 4k video.
Camera resolution is not linked to display resolution.
And it doesn't play it back in 4k, it plays it back at approximately 80% size.
The maximum that YouTube will show is 1440p videos as only mobile devices with 4k displays will get the 2160p option in the app.
At the moment I believe the only mobiles with the 4k YouTube option are the Sony Xperia phones.
The exact resolution of S10 is 3040x1440 which makes the S10 a 3k device.
I'm stumped ... my device plays 2160 just fine on Youtube ... Here is a screen shot. But yes, makes sense about the camera not being linked to the video playback of course.
However, on a device with the 4k display, the app will show a 4k option of 2160p.
On the S10, and any HDR screened device.
It will ALWAYS look better in the app on max resolution in HDR, using browser for a 2160p stream that doesn't fit 1:1 and non HDR, is not beneficial at all