Does this plant have some kind of nutrient deficiency? Yellowing first true leaves.

[bluter]

Yes, the first point I agree with. The question is why did it happen?

In addition, we established the second point three days ago, at which point I took actions that seem to be paying off (general application of nutes, with extra calmag way above and beyond the recommendations by Remo Nutrients, and now still above but backing off a bit back to recommended levels).

did you build the soil or purchase it? if you are in a true super soli or LOS the only thing you add to grow is water. you don't feed those soils. makes me wonder what you are in reality doing.

So, "is underfed" I think time will tell. I agree it "was underfed".

it looks to be continuing. in soilless you should see a marked improvement within 24-48 hrs. i'm not seeing that. soilless runs quick.

what is your soilless - coco? if you are feeding them all the same it won't work.

I find light stress plausible because of the coincidence of when the browning happened to all plants at the same time (increase in lighting) and then seemed to stop at the same time (when the lights were dialed back).

it's definitley not light stress. the plants don't show anything associated with it.

dialing the light back just slows everything down including development. it's not actually fixing anything. long term it can result in weak weed.

it's ok to dial it back while fixing an issue. what you really want is running the plants at the right balance with as much light as they can take. light is the number one most important factor, followed by nute levels and environment.

good news is they really aren't too bad and should fix up nice.