After a couple grows doing bottom-feeding I got lazy about top-watering once every few weeks to flush out any salt buildup, and I think that's the root cause of the problems I'm seeing this time. I watered PP1 to runoff to check, and the input of ~1300 us/cm had runoff around 2900 us/cm, so there's definitely some salt buildup in the coco. It's probably throwing off the pH, and each plant may be reacting to that a little differently. I top-watered all the others and removed the runoff. May be too late in this grow to make much difference (I'm probably harvesting in a week or so), but I should be more consistent about that next time.