Hey Growmie ...
Just from my experience with autoflowers, this is usually a nutrient deficiency or toxicity from the acidity or alkalinity of your soil (PH) or from nutrient concentrations being too high or too low. I’m thinking it's most likely a phosphorous deficiency or maybe toxicity from your PH in the soil being off, making something too available or from something too concentrated in the feeding regimen or it has built up in the soil.
Hopefully, this was helpful and remember ..."Happy Growing" and "Free your mind one puff at a time".
Mr_Weeds_Autos.