I had this issue. I assumed it was heat related, as the laptops can get real hot and sometimes I’m sure it doesn’t shut down properly when you close it - on mine the marks corresponded with the top bit of the keyboard, where it gets hottest.
Brought it in under Apple care and it was replaced (along with the keyboard, rolls eyes) no questions.
Brought it in under Apple care and it was replaced (along with the keyboard, rolls eyes) no questions.