From that stand point, he was burned with lava. Looking at Vader’s bald head shows the damage removed all his hair. Eyebrows are hair. So to me it would make sense for the removal.
It does make sense if you simply apply the same rule to all facial hair regardless of actual damage to the area. You are right that the top of his head is damaged but his eyebrow area is not, so even if his eyebrows were burnt off due to proximity to the lava, they would grow back.
I know this because of the fact that my cousin once had all his eyebrows burnt off from lighting a bonfire with petrol. He let the fuel sit too long before lighting it, allowing the vapour to gather around the bonfire stack and when he lit it, the vapour exploded and went up in a huge fireball, burning my cousins eyebrows right off. He still has eyebrows today though as they simply grew back 😉
If Vader had sustained the same kind of damage to his eyebrow area so as to stop them growing back, I doubt he would have a face left.
OT.com, the only place you’ll find people debating whether to keep a character’s eyebrows or not LOL! George simply shouldn’t have removed them in the first place, in which case we wouldn’t be having this discussion.
.Val