To see the bug,you can summon an ender dragon.Set your gamemode to the spectator mode,click the head of the ender dragon(You can F3+B to assist you.),you will find that the dragon flew away and you are in the location at which his head was just now.You do not move as the dragon move.If you press shift,your location will become to the location where his head was when you pressed shift.

The video is too big to upload,but the bug is easy to reappear.Thank you.