Fan leaves yellowing fast. Help please.

Ok so over the last week or so I've noticed my fan leaves on most of my plants start to yellow real fast. I know they should yellow towards harvest but I have almost another month of flowering so this is too soon. Also the top yellow leaf in the picture literally turned that yellow over night. Any help here would be a life saver. I know it looks like nitrogen deficiency but I've been feeding them so don't see how it could be. Soil ph is around 7 it might be a tad higher now. Again any advice will be duly appreciated.

A close up

I've also noticed that the part of the fan leaves that were overlapped by another fan leaf
Do not have the yellow colouring on them only part of the leaf exposed to light. I'm totally confused.

You can see here in the two images

i am experiencing the exact same thing right now. i plan to flush all salts & restore balance, the feed with half the recommended dosage of ff grow big. it has worked for me in the past on previous strains.