Enderdragon Bossbar not showing/ They are glitchy

when using /summon EnderDragon it wont show its boss bar and it seems to act odd, like it will stay still then start moving foward.

https://www.youtube.com/watch?v=D9GPSc_Q_Ts&feature=youtu.be
a video to show what i mean. (ignore the last little bit i forgot i was updating a driver)