Is this a magnesium sulfate deficiency? Or light/ nutrient burn?

Hey Growmie, Looks like a bit of leaf stress going on there for sure. From the spotting and crispy edges, it could be a few things, possibly light stress if that leaf is sitting close to your light, especially if it's intense or too close to the canopy. It could also be slight nutrient burn, especially if you've been feeding heavily or not checking the runoff EC. Magnesium deficiency is a maybe, but it usually starts with interveinal yellowing before you get those rust spots, and this looks more like external edge damage. I’d suggest checking your light height and measuring your runoff pH and EC just to rule out salt buildup or lockout. If you’re not already using a cal-mag supplement, adding one can help, especially if you're in coco or using filtered/RO water. Keep an eye on the new growth, if that looks good, your plant's probably bouncing back just fine.