i think you might have stressed it too much with the transplant, set it off to the side a little, so it's not getting full light, and feed it lightly for a few days. that's too localized to be heat stress, and happened too quickly.
looks like you're setting them outside during the day? if so, you need to do that in stages, going from your tent to full sun all day will burn them pretty good, you want to give them an hour or less the first day, work them up to a full day over a week, at least