I have the same thing on one of four plants. PE Bench Sitter is my runt. Far as I can tell, it was high heat and humidity for a couple days. I know it was not the light as PEBS was the farthest away. I can't say with your setup.
You cannot get rid of what has happened. After you get your watering dialed in, keep an eye on newer leaves to see if you keep getting the lily pads.
I see you have a dehumidifier going. The downside is anything electric makes heat. Do you have control of your temperatures?
Nah, no dehumidifier. My "lung room" humidity is at around 50% all day every day, I only have a smart humidifer and a smart exhaust system that keeps everything in the best possible VPD range. I can't exactly raise temperatures that much, I have to live with LED heat and ambient temperature, which gives me a stable ~75F or 23-24°C at daytime. Heat stress surely can't be a problem in this case.
They're inside a 2.3x2.3x5ft tent, it's a strange size. 70x70x160cm to be exact, not the normal 2x2x5 tent. LED is the MarsHydro FC3000, currently at 20% light intensity and at around 2ft of height. I do have an emergency tent if things get ouf of hand, it's 100x100x180cm. Bigger tent means lower temps though, as the LED can't heat up that big of a space that much. Good high temps are always hard to reach, so I use the smaller one for now.
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DLI and nutrient schedules are generally a problem right now. See, the big plant has a VERY complicated past. It's hitting a bit over one month now. It all started in organic peat substrate (BioBizz), and I couldn't get the watering right, always overwatered. It was so severely stunted, the newest growth almost became white yellow, it just completely stopped growing for 10+ days.
I then washed the plant out of the substrate (really easy with peat, it just disintegrates) when it was stuck in growing it's first set of 3 finger leaves. I repotted into another pot, with fresh substrate, and f-ed up again. That plant went through lots of stress early on. I then got coco and all that stuff, and started another attempt with a new seed. It grew so nicely that I thought "f it", and once again ripped the plant out of the peat, washed the roots off with water and some peroxide afterwards, and threw it into a new pot with coco / perlite too. And that's when it took off REALLY fast, but sooner or later the leaves just started drooping daily. I trusted myself with this crazy process because I do own tons of plants, literally TONS of plants, even trees and all that random stuff, I do have a green thumb and know how to handle roots and emergency "surgery", but I am dumb as a brick as soon as I touch weed.
This plant is pretty much exactly one month old now, with lots of chaos in its past. But it's living now, and I feel like it's getting back to health already. Problem is that I don't know which stage it is in, as it was stunted so severely over weeks.
Wind burn was a thought too, I already raised the oscillating fan so it just throws a light breeze over the plants, moving the leaves slightly around, but not severely. I also just noticed that the small new growth on the bottom (where light doesn't really get through) also has those serrated edges curling a bit up, so it can't really be the light.
