I learned this so long ago I couldn't tell you where I first heard it. Maybe a special on the making of the original series(first three). This one is fun trivia.
In January 11, 1977, a day before he was set to shoot one of the final scenes needed for Star Wars Episode IV: A New Hope, Hamill was involved in a car accident that caused substantial damage to his face. A double was used for the scene of Luke racing across the desert in his landspeeder while Mark was hospitalized.
.......
An in-universe explanation for his scarring was developed for Star Wars Episode V: The Empire Strikes Back where he is mauled by a Wampa, which seemed to be a pretty good way to many to allow him to continue, even with the scar.
Mark Hamill (Star Wars Wika)
I do remember the Wampa scene; it had him hanging upside down in the ice cave.
Well, that’s a good writer there, that can blend something like into the storyline.
Next time ‘the Empire’ is on watch the first 20 minutes (that’s what DVRs are for, they are magic) and see how they block Hamill’s face until after that scene.
Trivia can make it new again.