as far as i can tell this is healthy, but what do ...

Hey there RAW, so basically it's a magnesium deficiency. It's actually from the pretty heavy light schedule, which causes the plant to need more Mg for photosynthesis. You can tell from the red stems and the yellowing on the edges of the leafs. these are the signs of stress. Basically it's down to your nutrient schedule also, you're adding NPK part A & B , and four different additives, so what you're missing is the cal mag , light mix is actually a soiless medium, much like coco, but it's peat moss and retains moisture for longer. You need to add cal mag quite urgently, and fix this before entering flowering. Do this and the problem will stop spreading immediately and the plant should recover within one week or so. Hope this helps ! 🚀