This isn't technically a bug, but rather a mechanic that needs updating, probably a human coding error through oversight and isn't intentional in design. 

When a fireball (from a Ghast) is fired and hit by a projectile (i.e. an arrow, snowball, etc.) the fireball is sent back to where it came from - usually. I've noticed that the code that decides which direction it goes in is based on the direction in which the player is looking. 

For example, if a Ghast shot a fireball at me directly south (I'm facing North) and I shoot the projectile mid-air with my bow and quickly face East before the arrow hits the fireball, the fireball will travel Eastward regardless of the angle I hit the fireball. I know physics isn't necessarily realistic but it just doesn't make sense why this would be an intentional game mechanic. Possibly outdated code but I think it should be updated in the official release of 1.16 since it is, in fact, the Nether Update.