Windburn?? SS Chronic & HDF

Hi I'm growing a Serious Seeds Chronic and a Heavy duty fruity plant in a 4x4 tent. Both are a little over 3 weeks into flowering.

I have 3 fans of which I have had 2 blowing on the plants at an upward angle from the base of the plants. I notice the problem on the bottom leaves and other leaves which are in the direct path of the fan. Could this be windburn?

I originally considered ph, overwatering, nute burn, cal/mg def. but my Ph
runoff is about 6.5. I am watering about every 6 days till only a little comes out the bottoms of my 4gallon conatiners. I started giving 5ml/gal of cal mag 3 weeks ago at first sign.
I have however been giving full strength fox farm trio nutes and im in FFOF. No burnt tips though.

I guess of all those, I am hoping it is windburn.
My Temps are consistently 73-75F lights on and 66F lights off.
My RH hovers around 50%.

Windburn. Lol.

Your plants are fine
As plants get older they realize some of their old leaves just arent cutting it any more. They arent making energy for the plant anymore, just sitting there being stupid leaves. So the plant says "Screw you, stupid leaf!" and starts to eat it away from the inside out. Literally, the plant sucks all the nutrients out of the leaf, killing it's life force. The plant then uses the life force from this leaf on new growth on the top of the plant, where those leaves will be producing energy for the plant and not just draining the plant of all it's sweet sweet energy.

Your ladies seem to be loving the FF. Their coloring was a little light, so possibly there was a cal/mag deficiency which you were able to treat. That was going to be my only other suggestion - as with the FF trio of nutes (which I use as well, fantastic stuff) they tend to go a little light on the calcium and the magnesium and its always helpful to add a little of that in. Maybe throw in a tablespoon of epsom salt with your next feeding.

All that growth and this is your first leaf the plant is sucking up? Not bad at all man

My Conditions are the same but now all my fan leaves are quickly dying and falling off. They start out looking like the pictures above then get worse. Any thoughts?