Basing this answer on and assuming your run off is coming back good? Not sure if the 6.6 is for feed or RO but if your soil pH is good that's what I'll focus on👍.I do believe you're looking at a very slight nute burn. You may not be over feeding them, necessary, but with the high heat and lower lower humidity, she may be eating and taking to much too fast. The humidity probably needs to be a bit higher if you're able to bump it up. Roughly 60% RH. Lastly, I used a Phlizon 1200 in my first grow, and I feel 26" is good, but as precaution, just for the time being, try backing it off by a few inches and see if she stresses less.
I don't feel it's a big issue and it just caught a little too many nutrients, there. Good luck in the coming days, I'm positive she'll grow out of it👍 Cheers