I have the same problem, it's the same model and I noticed the problem a week ago. I tried to restart the TV several times and connect to another Internet LAN or WiFi but always the same. What I noticed is that I have the same problem on the Nexbox A95X Android Box on the official YouTube app. Since I am from Croatia, I contacted support with us and they told me that the problem was with the YouTube application. If anyone has solutions to the problem bothering a colleague in the post above, they would be grateful to get in touch.
I have the same issue with my Samsung UN65JU6500 after about a week ago it received an update. YouTube is locking me at 720p, while other streaming apps like Netflix run perfectly fine. It does sound like a problem on YouTube app's end. I can pull up a video in 1080 on my Galaxy phone, but when I cast it to the TV it downgrades to 720p and wont let it go back up to 1080.
It's the same problem for me. The problem is in the video which is at 60fps. No one has responded from Samsung yet and YouTube has not corrected the problem. If anyone knows what the email from YouTube is I am willing to write them an email.