What’s your water PH running at?
Yeah I think that’s a little low, and with enough time you will see affects like you’re seeing. FWIW I run my PH closer to 7. Nutrient uptake only happens in a PH window otherwise it gets “locked out”. Here’s a chart from growweedeasy.com
Ok, that makes sense then. Maybe someone more familiar with coco will chime in. Good luck I hope you get it resolved quickly.
I just finished flowering a Chernobyl that kinda did this to me, too. In my case it was almost certainly related to low pH, turned out I was feeding at 5.4 unchecked for a couple weeks 
At first I thought it was rust fungus and I was FREAKING OUT, but I still go back to it being low pH and probably a bit of nutrient burn as well.
That looks like an odd one to me, let’s call in one of the resident experts… @George, what do you make of those photos?
Cheers
G
That one really bad leaf even shows some magnesium deficiency “tiger striping” 
I mean, as long as the problem is isolated and not super widespread throughout the plant, that’s a plus IMO
Thanks for tagging, according to these pictures it might be Sulphur deficiency, some Epsom salts may help to correct it … 
Thank you! I think I probably didn’t add the needed florabloom when I was mixing nutrients at end of veg. The flora bloom should have phosphorus needed as well!
Try running your PH near to 5.8. I run two 160 plant rooms at work that are in pure coco. We PH to 5.8 and things grow wonderfully as long as you match your fertilizer strength to your environment and light level. That should at least rule out the issue being the PH.
I also run 5.8 as well in coco. Try shooting for that OP.
Some screenshots of my recent fuckery
First things i checked, were flowers rotting (no) and does the rust rub off (also no) and clearly it’s not just nutrient burn. After checking my feed pH after not really checking it for several weeks (doing formula by memory) turned out pH was way low for at least 2 maybe 3 whole weeks.
Interesting! Still lovely nugs!
Is this at the root level or in the reservoir? As they will differ
Striping and some of the yellow almost white coloring is usually a sign of pH imbalance… I’ve had it a few times and should correct itself overtime.
, but colour and shape doesn’t likely match …
Could this be some sort of lock-out from running coco in Autopots? Coco seems best on run to waste but in autopots there’s no waste runoff.
Put my vote down as calcium deficiency