So the worst issue with ghasts right now is their aim. They're definitely aiming half o' block above the player's head, that makes deflecting fireballs literally impossible sometimes, especially combined with the fact that they like to float above you, so fireballs are targeted not even in you, but one block behind you. I think they always aim above the head, but misses occur only if ghast is significantly above the player, distance doesn't matter. If ghast is directly above the player, it shots one block behind the player, if it is some distance from the player, but is not directly above, something like 45° relatively to the player, it aims ~0.5 blocks above the player and misses.
Deflection of fireballs that are aimed to the player by ghast that is at an angle of ~60°-90° to the player is impossible without moving.
To replicate:
Get yourself an unfriendly ghast by any means
Get yourself below it at an angle of 45°+
Make it shoot you
Note how fireball consistently goes above player.
Note: it might still hit you with an angle of 45°, but should always miss when right above you (90°).
Video with testings: https://youtu.be/188D9cjzooA
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I could confirm, Relates to MCPE-35202
Ghasts aim at the middle of a player or snow golem when at the same Y level, but above when at a higher Y level and below when at a lower Y level. A possible common cause is that the target is actually offset by a block or two in the direction that the ghast is facing--in other words, behind the target.
In the videos below, particles trace the server-side position position of the fireball by using /execute...run particle...
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