If using Youtube App, the video will split in Portrait mode, landscape mode is ok.
if use chrome to visit Youtube website, the video is ok.
Any luck with this problem? I observe the same on plain Android 6 on M64 in portrait orientation, however playback doesn't work not only in YouTube app.
I had the same problem... so in case of rotating screen to landscepe, in all videos there was green line.
The solution is simple ... go to Developer settings and turn on hw video overlay.