That is physical damage, or there was a problem for a very SHORT period of time while that band of leaf was developing- like, it hit the light or something. It's fine now. But it won't be for long if you keep taking your pH down that low in that soil
Set your pH to 6.7 for feeding in peat based soil that has already been treated with acid for so long and keep on top of your magi-cal. Lose the superthrive on day 1 of flower.