What could cause this burn mark?

It's all good 👌 their just the jump starter leaves, the plant has no use for them once big enough. All looks good carry on as you are .

Hey mate, Dont worry about the cotyledon. They die off as the plant matures. You plant is looking great. Keep up the good work :)

Hello, good afternoon! I would say not to worry, since it's less than 1% of the total leaves with that color. This is common when the plant is over a month old, and those colors show up on the older leaves... it could also be from burning, as you mentioned in your comment, but it's nothing to be concerned about... they're doing great! Cheers!

I wouldn’t read into anything much concerning the cotyledons once you heave established leaf sets already.. as long as your actual leaves are looking good, and they do appear to be, you’re in good shape!