Hey friend,
This is kind of a hard one to call.... the main thing is you're ready to harvest...so flush and cut.
I'd personally think of a couple things, 1 maybe run an 18/6 or 20/4 the next go, it could be light irritation. 2 double check your ph pen, use some calibration powder to get a solid solution like 6.86 check that the pen is reading 6.8 not something different. I've had a few grows get wildly crazy yellow and damaged leaves from the wrong ph and pens that lost their accuracy.
Happy harvesting. ✌️