There was no mask burn. The damage was done by the explosion. You're confusing the scenes where Kristoff gets subjected to the Remembrancer.
If you look back at the Lee/Kirby origin, the monk is handling the mask with his bare hands. And immediately after, Doom is speaking without exhibiting any reaction.
In Secret Wars #3 (2015), he does take off his mask
But wasn't John Byrne's story the opposite? I remember him saying that the explosion just caused a little scar, but Victor was so upset over being "disfigured" that he later put his mask on when it was still hot, which caused the major damage.