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
Wear a full set of Thorns armour
Spawn in a skeleton and let it attack you
Notice the skeleton almost always takes damage from your armour ✔
Spawn in a ghast and let it attack you.
Strafe around a little bit so the fireballs don't hit you as directly, but still let them hit you.
Notice that the ghast more frequently isn't damaged by your armour ❌
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This is very likely. I think the ghasts' fireballs sometimes explode when hitting the player but don't damage the player directly, if they don't hit at the right angle. More testing could be done, I think checking death messages between different ways ghasts kill me could help.
Update:
YES! This is what causes it! Ghasts have two separate death messages, "<Player> was fireballed by Ghast" and "<Player> was blown up by Ghast".
It is possible to get the latter while still being hit by the fireball, meaning sometimes the explosion damage is applied instead of the direct fireball damage, which is why the Ghasts don't take damage from thorns very often.
Affects 23w46a.
This bug is very easy to notice if you have fire resistance, since that effect makes entities immune to damage from fire-based projectiles but not the explosions they cause. (This is very counterintuitive and arguably a bug in its own right.) In the following video I have fire resistance and the tick rate is set to 10.
I think this is because taking direct fireball damage will carry out the thorns effect but taking explosion damage from the fireball doesn't carry out the thorns effect