Necrosis/chlorosis very rapid onset

VPD is too high. High VPD causes rapid transpiration which means water and nutrients are taken up faster than the plant can process it, which causes excessive nutrient build up in the plant tissue which leads to burn. I see the leaf edges are raised on your plants slightly. Thats a sign of high temps and low humidity which = high VPD If you are in veg then your RH should be like 65% to 70% Coupled with mid 20's temps. That'll give you a VPD near 1.00 which is pretty good for vegging weed plants.

@Organoman funny, no nutes have been added at this point of growth. @Chriss42081 VPD for this stage is way off for sure, temps are between 22c-26c with 40%-55% RH

Firsly, growing at 24/0 is not good...........18/6 is about optimal. Further, using "nutrients with numbers like 6-20-35 and 20-20-20 is also REALLY bad. Your plants are no doubt suffering from nutrient toxicity. Get some dedicated cannabis nutrients with numbers more like 6-4-9.

Chk on vpd. Personally having issues as winter is coming and things are dryer. Also chk ur CO2 if possible. For me it wasn't possible. So I thought. Do I have enough fresh air in this place?. Answer was no in late fall winter. So I added a CO2 bag and watch my vpd close and have less issues currently. Another really good way to tell if it's CO2 if you can take a scrawny plant and put it outside you'll see this thing come back to life instantly if it does it needs air they're being suffocated I did a little experiment myself. When mine curl vpd is off.