Right when I open the app and choose a 1440p video I have to go in to the little gear option thingy and change it to 1440p. From here the video gets choppy and lags but if i go to another video and YouTube saves my 1440p selection and it automatically starts the video at 1440p where I don't have to go in and change it the video has no lag.
I noticed same thing on first week , but i found that high gfx themes eat up lots of display/video memory so the video stutters alot , if you have a theme from samsung or any launchers try to disable them and check you ram usage ,i hope this should help.
After having some normal theme my phone working great with 2k content.