He didn’t just get a burn, he was on fucking fire. His clothes seared to his skin. If you look at him in Jedi, he looks really good for a burn victim.
So, we’re supposed to assume that his appearance in ROTJ represents the physical damages he incurred prior to the mask, or that he naturally healed very slowly from those injuries, or that his body/face took a long time to get repaired through medical means.
I think Lucas intended the first, because their was nothing to imply that he was trying to get his face fixed in the previous films, and probably he didn’t want Vader’s countenance to look too gruesome for the younger demographic he was aiming for (see The Ewoks).
Yes, he was on fire, so you’d think that the Emperor would have put him in some kind of therapy/rehabilitative/medical procedure, or something. I think the tech seen in SW would mandate that. Instead, he’s instantly put into the robotic suit. You’d think he’d be put on some kind of respiratory system, while trying to at least repair SOME of his injuries. Which is why I don’t think it makes sense with what we know of Star Wars.
They can recreate the look of Luke’s hand, seamlessly, when it wasn’t on fire, it was completely gone, but Vader’s charred body right away is put into a suit.