I summoned a Fireball with the Invulnerable tag set to 1 and encounter some very strange behaviors of the fireball.

First, the fireball is completely invisible and only seen for a slight time when first summoned. (less then a second) The fireball still explodes and deals damage
If you attack the fireball, it is rendered as flying away, but it actually still explode where it would have without a hit. When these rendered fireball fly to far away, they'll just get set to the starting position, even if the real fireball already exploded. Relogging/Reloading seems to fix them, but sometimes they'll come back and start flying in the loop again, but will not fly in the direction they previously were.
These client-side only fireballs do not explode, even if the hit a player or a wall. The fireballs may disappear a short time after hitting them.

I used this command to test:

/summon Fireball ~ ~-2 ~ {ExplosionPower:1,direction:[0.0,-0.5,0.0],Invulnerable:1} 

(Flies downwards)

/summon Fireball ~ ~-2 ~ {ExplosionPower:1,direction:[0.0,0.0,0.0],Invulnerable:1}

(Does not move, hitting hit sends it flying client side only)

(Changing the ExplosionPower or direction does not change the behavior.)

I tried providing a video, but my pc and internet speed seem to not allow me to.