As a brief aside:
In MCPE-54306, a fixed bug report for 1.12, it was stated that ghasts seem to only have a 20 block range instead of the 100 that they are supposed to have according to the wiki. Now, in 19w46b, you have to be standing inside of a ghast for it to fire, unless it already noticed you from being within this range.

Main problem:
Parrying a ghast fireball with an item with knockback will cause the fireball to skew off in some weird direction, seemingly on some exponential curve (perhaps parabolic, if the acceleration is changed rather than the motion, perhaps?). It gains verticality and does not even vaguely approximate the direction it was hit in. I have also noted it to teleport from the ghast's mouth to the player but that could just be my environment as this does not happen every time.

Steps to replicate:

1. open a Minecraft 19w46b world
2. Run

   /give @s iron_sword{Enchantments:[{id:knockback,lvl:8}]}

1. Summon a ghast with a command or an egg so that it will be able to target you, finding a naturally spawned one will probably not work unless you get within 2 blocks of it.

1. Wait for it to fire.
2. Hit the fireball with the knockback sword.
3. Watch in disbelief as the fireball arcs towards the heavens rather than goes in the direction you hit it.

If 'ghast' is supposed to be capitalised, I apologise for mislabeling a proper noun.