Looks like phosphorus from overwatering to me. The leaves get stiff and darken before necrotic blotches appear. Also looks like nutrient burn.
If your temps are too cold and your watering to much and often this will happen.
Let that soil dry out quite a bit. Autos don't use alot of nutrients compared to photos. However, they have alot to do in a short amount of time so just make sure you have a well rounded nutrient on hand and use it very very sparingly.
Get your day/night temps in range and stable.
Keep humidity at 50-60% and lower it to 35-40% when your in full flower.
20/4 is a good day/night schedule. Last 3 weeks put them at 24/0
I'm a photo guy and grow almost exclusively. Autos have their place but in the last 12 years I've only been happy with 1 auto out of probably 20.
Also IMHO you'd be better waiting the same amount of time to harvest and just grow photos. Save on some electricity in 12/12 flower and crop out waay heavier.