Man am I glad I found this thread!!! I'm just starting week 4 of 12/12 and my plants are doing the EXACT same thing!!!! Buds look awesome, but the fan leaves (not all of them thankfully) turn yellow, then start getting the brown/rust colored spots. I've been reading threads for the last 4 days trying to find a cohesive answer to this problem. At least now I know I'm not alone on this issue.
I don't have my current Ph (my puppy destroyed my meter and I haven't got another one yet) but I've been alternating using AN's Big Bud one watering, then molasses with the next watering. I definitely have a nitrogen deficiency also, so when I watered this morning I used FF Grow Big instead of the Big Bud to see if that helps. I've been getting REALLY frustrated with this because everyone has a different answer --- but there are some very common themes: Not enough nitrogen or calcium (I guess I've got to see if I can locate some CalMag tomorrow). Hopefully between all of us who are having this issue we can find a cure!!!