This looks to me like possibly a case of slight over-watering... since the bottom leaves are only effected and over-all you've got a tad bit 'claw-leaf' happening (jagged edges curling down slightly). If you're sure your nutes are correct and the water/soil pH/ppm is normal - i'd try cutting back on how often or how much water you're giving them.

Make sure you give your plants enough time to fully use up the water you've given them before giving more - some strains like to be a bit dry for a while before getting more water, and this strain maybe specifically likes it that way during flowering..?

One other possibility could be the genetics - i've heard that some plants' genetics can cause the lower parts of the plant to die off sooner that others.

...hope that helps!