It's the Youtube app. It's bs in terms of coding.
I had this problem with my iPhone and now on my Samsung s10e. Where the app just plays in 480p or 720p (at best), if I set it to automatic resolution.
Despite having full HD screen (or on iPhone at least above 480p).
The app just doesn't care.
You need to set it manually sadly.
It doesn't care if you have: full HD screen and you are on 150Mbps wifi.