Heat, RH or overwatering? Or alltogether?

I would say you are right, the heat coupled with low humidity without a doubt. You could try and put a screen (like one from a window) overtop of the young plant to break up a little bit of direct sunlinght. That paired with a humidity dome (clear plastic cup or water bottle) over her to keep the RH up. Should be around 60% in this week for sure. Don’t mist it, that is going to create burn spots on you leaves especially if you do it just as it’s getting hot outside in direct light. If this plant is indoor I would say you are going to need a humidifier and some form of cooling ie air conditioner. The other option would be a bowl of ice by you intake hole or fan. Hope that helps. Good luck!