So this is a weird one.
Steps to reproduce:
1. Place a repreating Command Block
2. Input command: /execute @e[type=Arrow] ~ ~ ~
/summon Fireball ~ ~ ~ {direction:[0.0,0.0,0.0]
3. Power the Command block
4. Place a dispenser
5. Put arrow in the dispenser
6. Power the Dispenser.
Observe that arrow DOES hit the Fireball, since hit sound can be heard, and the arrow disappears.
Same goes with arrows summoned by commands/command blocks.
Follow steps 1-3.
4. Input into the chat /summon Arrow ~ ~ ~ {Motion:[5.0,0.0,0.0]}
The hit sound can be heard again, the arrow disappears too upon hitting the spawned Fireball.
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Previous comment has been deleted as the behaviour is EVEN STRANGER than that. When I added a "direction" tag to summon command it didn't affect anything. I tried shooting the arrows with the delay again and I've found out, that the fireball is taking THE DIRECTION PLAYER IS LOOKING AT when hit. I'll try to attach an appropriate video soon.
I've uploaded it to Youtube to preserve quality of video. https://youtu.be/mteYszrAkhU
My guess is, when hitting the fireball with an arrow, the arrow triggers the same event as if the fireball was hit by melee attack. It would theoretically explain the issue, because when an arrow is summoned, it doesn't have any "owner", so the direction would be nullified. And dispensers... I think they are not valid owners, or they just summon an arrow without any additional tags.
Attached a Test world for testing the fireball behaviour. Repeating command block checks for any existing arrow and outputs to comparator redstone signal, to delay summoning of the fireball. Just shoot an arrow up in the air and look somewhere else.
Is this still an issue in the most recent versions (currently that is 1.10.2, or 16w42a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time.