Your body disappears in the water in f5 / third or second person on camera

This is something that has happened to youtuber doctor nok and in his video he showed that his character disappeared under water, he was in the 20w06a. He starts talking about it from second 25
link of the video: https://youtu.be/nxuD3yq3Hzo
 
(i cant put the video downloaded  because its too big and i cant compress it)