i would do a good flush on the next feeding then try zyme ://growdiaries.com/nutrients/green-planet-nutrients/zyme that will fix root problem then your led must be 24-26″ (60-66cm) or more from lady

It could be a genetic issue not being resistant to heat or stress..

Lights are too low. A lot of strains can deal with lower lights but some brown and curl. Can't remember which blog of yours I was last reading, but I think you were using Bestva 2000 LEDs?

Run Hydroguard with your nutrient line.

I would suspect if your reservoir is 82f like the room temp the water cant absorb enough oxygen. I stay away from dwc for this exact reason. Anything above 72 and the funky stuff likes to start breeding. I use hydrogen peroxide to fight this. 2 tsp per gallon of a 3% peroxide solution will kill any bacteria and also naturally oxygenate the water as it breaks down. You can go up to a max of 3 tsp per gallon as a last resort. Hope this helps!