Are you copying a link and pasting it? Mine didn't work when I did that. Mine only works if I:
-open new private tab in safari
-search YouTube into google
-click on YouTube website from google
-search the video title
-play video FULLSCREEN
-press off button/home
-press play on control/lock screen.
If I paste a link into private tab it doesn't work, also make sure all your other private tabs are closed before starting a new one to make this work.
I believe this is a thing that YouTube has done to anyone who has an account or who visits YouTube through a direct video link. That's why if you use private browsing (not signed into YouTube) and visit YouTube through the normal route then it 'should' work. What a pain in the bum google, stop messing people around.