Having similar issue with heat stress, they don't have any "waxy" looking parts on there. I did have a temp jump to 88 one night, but it usually stays 74-76 40% RH. I'm using a 400w HPS and I have them 3' from the light. They are still recovering looks like this pic. Little to much N last watering but they didn't burn. I don't think what you got is heat stress.
I think you're right. I'm wondering if too much light at too young an age did it. It's a bigger plant but it's just over 2 weeks old. Also as a sprout the temps were 69°F - 70°F, a week or so later I raised to 75-76. Maybe that jump stressed it out? For yours I would say 40% RH is too low. Mine is 50-56% and I THINK that's a tad low for early veg. Sent from Los' Santos
Just moved lights to 24". I kept my arm across the pot and after a full minute my arm was heating up. These LEDs give off a very radiant heat that is not noticeable for a minute or two. Sent from Los' Santos