Ghasts have to hit you in exactly the right spot to deal fireball damage to you, or else they will damage you with explosion damage instead. The effects are thorns armour working a bit weird (not always applying to the ghast even when it should) and the death message for a ghast killing you sometimes saying "<Player> blown up by Ghast" even when it should say "<Player> was fireballed by Ghast"

The way I first found this bug was when I was in a soul sand valley biome with Thorns armour. Thorns damage is random, meaning sometimes a mob won't take damage from a player wearing even a full set of Thorns armour, but if you have Thorns III on everything, it almost always damages the mob that tried to hurt you. However, ghasts seem to be a lot more hit-and-miss when it comes to whether or not they damage you.

Steps to reproduce

1. Wear a full set of Thorns armour
2. Spawn in a skeleton and let it attack you
3. Notice the skeleton almost always takes damage from your armour
4. Spawn in a ghast and let it attack you.
5. Strafe around a little bit so the fireballs don't hit you as directly, but still let them hit you.
6. Notice that the ghast more frequently isn't damaged by your armour