Hey gardener! It's kind of weird to have that on just one leaf, but it might have happened because of the training. Don't worry as long as it's just one leaf. Good luck to you!
Since it's only 1 leaf no need to worry about it. Could be caused by spilling on that leaf or it simply touched the wet soil during training or watering. You're doing a great job though by the looks of your plant. Keep it up. Hope this helps. Happy growing ✌️👊