Within the OT alone, GL always conceived of Vader as a character who fell into a volcano and gets burned even before ANH.

https://www.rollingstone.com/movies/movie-news/george-lucas-the-wizard-of-star-wars-2-232011/
It’s about Ben and Luke’s father and Vader when they are young Jedi knights. But Vader kills Luke’s father, then Ben and Vader have a confrontation, just like they have in Star Wars, and Ben almost kills Vader. As a matter of fact, he falls into a volcanic pit and gets fried and is one destroyed being. That’s why he has to wear the suit with a mask, because it’s a breathing mask. It’s like a walking iron lung. His face is all horrible inside. I was going to shoot a close-up of Vader where you could see the inside of his face, but then we said, no, no, it would destroy the mystique of the whole thing.

So I guess it would make more sense for his eyebrows to be burned off.