EnderDragon glitch when summoned

EnderDragon glitches when summoned with the /summon command

What I expected to happen was...:
The EnderDragon should have been summoned and be like a normal boss battle

What actually happened was...:
The EnderDragon had no health bar and didnt lose health when it was attacked

Steps to Reproduce:
1. summon the EnderDragon using the /summon command in any dimension
2. ...
3. ...