Hello ! Well it's looks like a phosphorous deficiency to me , the brown necrosis on the leaves are a sign of P def. But as your plant looks healthy and only one leaf is touch then I think you had some water on the leaf ( 2 or 3 drops ) which created a loup effect and burn the plants. I hope it's not a phosphorous def but it's too early to really say I'm 100% sure. Hope it will help you đđ