It is enabled but youtube senses that picture in picture is activated and closes itself or pause. This is youtube doing on purpose to prevent people from using picture in picture mode. Its the same reason why they dont vant to implement such features in youtube app. To serve you adds that you have to watch.

I dont like youtube for other reasons anuway, i like Lbry on desktop and odyse on phone. But since they are not that popular i use youtube still beside them..