OP - I’m using Nutrition-Bloom Plus, and following instructions. Ferting every watering, but not OVER watering (from what I can tell). It actually started happening in veg phase. Just half of the leaf, as in photos. I have four different strains, and all are showing similar discolouration. I’m assuming it is a deficiency or possibly an earlier pH imbalance, but of what nute, I don’t know. I don’t believe it is strain specific, and definitely not normal. Thanks.
edit - No temp fluctuations, either. It hasn’t dropped below about 15C (60F).