In 22w19a, when I reflected ghast fireball, it just passed through the ghast and exploded, making terrain destroyed. However, the ghast did not get any damage. Also, the fireball cannot make some fire-immune mobs (just like zombified piglin) hostile. (For those mobs which can hurt by fire (e.g. piglin or wolf), ghast fireball can make them hostile. ) Meanwhile in 22w18a, these situation didn't exist.
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Can confirm. For easy testing I recommend using command blocks to teleport the ghast constantly.
This is likely introduced during the fix for MC-249888. Entity#isInvulnerableTo now returns true for if source.isFire() && isFireImmune(), and the fireball damage is considered a fire damage. Since ghast is fire-immune, it cannot be hurt by any fire damages (including fireballs) due to GhastEntity#damage early-returning on isInvulnerableTo call.
Yes, when I tried to deflect it at the bottom, it hit the ghast but it takes 3 deflected fireballs to kill it.
Tested this, and can confirm ghast fireballs are not killing ghasts but still exploding.