what day of 12/12 is it?
my thoughts for what there worth
its likely it could be natural in late flower.... if you've been leaching N intentionally (flush)
or if your nute profile is redcuing N during late flower
you might also consider N deficiency or PH being high
I understand your feeding water ph at 6.8 but that doesn't mean your soil is ph 6.8
over time medium ph can rise significantly higher than the actual water ph
most common in late flower since most people begin to restrict water to avoid mildew the soil doesn't get flushed as well so the waterings aren't enough to bring the ph down