The thing that bugs me is, if your nuits are weak, then where did the burning come from, and if your PPM meter is right, then your undiluted nuits are weak.
Yea, I tend to agree, ironing out what exactly is going on here, either a lockout or an actual bonafide deficiency, I'm inclined to think it's the former. Appears to have signs of potassium/calcium/magnesium deficiency all rolled into one.
If it's a lockout, (possibly even an over watering?) as I suspect, just coming off your regime a bit and doing some ph adjustment should straighten her out, whatever you choose to do, just watch them closely and respond accordingly. My usual approach in these type of situations is to be conservative, (flush or ph adjustment) tread lightly, then adjust from there.
The other thing that could be playing into all of this is (low) temps at night, and/or overwatering, especially at this stage. Is your pots getting the tops of the soil well dried before your watering? Have you been picking up the pot to judge water content and trying to give them what they drink everyday? or just filling them to the brim and saturating them.
I would try and make sure you've got some good air flow on the underside of that canopy, because your canopy is so dense, and it might be a wet mess down there. Looks like most of your issues are below the canopy.
Regardless, they still look fantastic, so don't sweat any of this too much. If there's some moisture or over-watering issues, fix that of course, but things overall still look wonderful.
