Ghasts with the {Invulnerable:1b} tag can still be killed if hit by a fireball reflected by the player. The expected behavior is that they don't take any damage at all.

The code seems to specifically override reflected fireballs to always do damage to ghasts and not be affected by invulnerability.

To reproduce

• Spawn an invulnerable ghast in a two-block deep hole to prevent it from moving: summon minecraft:ghast ~ ~ ~ {Invulnerable:1b}
• Switch to Survival mode, as players in Creative mode can damage invulnerable entities
• Attempt to do melee damage to the ghast or shoot it with a bow. It should not take any damage.
• Hit one of the ghast's fireballs back at it, which will kill it instantly.