Can anyone help sort out the issue I'm seeing...

First of all, without clear pictures of the affected leaves I can't be sure of what exactly it is. A deficiency at the end of the stretch, beginning stages of bud production is normal, if it shows on the lower leaves with dark / metallic blue spots and withering it is phosphorous deficiency, sometimes it also comes with red / purple petioles (stems attached to leaves); chlorosis (yellowing) starting at the tip and edges of the lower leaves with rust spots close to the edge of the leaves if it is potassium deficiency. If it looks like homogeneous yellowing with no spots, it's nitrogen deficiency. During stretch, some nitrogen deficiency is quite normal, since the plant explode with growth; P and or K deficiency after stretch is normal too, it just means that the feeding needs to be stronger or done more often). Your bloom nute is 2-4-4, I would up the dose or frequency, what is the EC / PPM of your feedings? At this stage I usually feed 1.3 EC / 650 PPM. As for soil run off PPM's, don't pay that much attention to it, it's just a way to see how exhausted your soil is and how much nutes your plant is taking related to your feeding. Mine starts at 10.000 PPM (yes 10 thousand PPM) and when it's at 5.500 PPM my plants show definitive signs of deficiencies and it is when I have to start feeding; at the end of the flowering stage, even with feedings, my run off PPM is between 1.000 PPM and 3.000 PPM, as long as you run off PPM is not increasing you are good to go.