at 71 days it's normal for either indica or sativa to yellow and drop some leaves, not a lot, but a couple is ok, also a sign that she is maturing/ripening.
if you are set on it being a nutrient deficiency, then I would say your pH is jumping to 7.4 because you are K deficient and that's the dominant element the plant is trying to take up. Root exudates and elements not being used will throw your buffers off and pH climbs. If the nutrient demand is not met and deficiency eliminated, the balance of nutrient uptake will remain off and the burn along with pH fluctuations will remain and just get worse. Once the nutrient deficiency is met, then the plant will resume it normal nutrient demand and pH will balance it's self in-between resivore maintenance.