rapidly yellowing top leaves

It looks as if it's only happening with the bigger older shade leaves in the photos you have provided with this question. Not every plant goes for 8 weeks during flower cycle, this could just be its late flower fade. Do you know the cultivars name? If so try looking up other diaries on this site for the same cultivar and compare their late flower stage with yours, try to find one from the same seller of the seeds so you can eliminate some of the deviation in genetics. Also environment plays a large role on expression of genetics, so when looking at other diaries make sure to check how close your environmental settings are to theirs.