Could this be some sort of light burn?

it's not light burn.. It's from Excess Calcium.. Some might give you the advice that you should "add" calcium when you are already adding cal-mag.. The thing is it's super easy to add too much.. and because your growing in soil you already most likely had enough nutrients to grow your plant for quite some time. You can water to run off and make things even more complex.. Or you can trust in your peat moss based soil and just use plain old water for quite some time..