It was a bit hard to tell precisely where on the plant(s) the leaves were but judging by what size they appeared to be I was guessing lower half, maybe lower third of the plant(s).
It is clear that it is happening between the veins. I could not tell if it started along the middle/main vein and spread here and there or just showed up randomly but it is between the veins on what are not new leaves so that means two likely causes are Mg – MAGNESIUM or Mn – MANGANESE.
If Mg. lower leaves will yellow and may even turn white while veins remain dark green. Blades die and curl upward.
In three for sure and maybe on all four pictures that looked fairly like part of the symptoms. While there were not many places where there were brown spots on both sides of veins of the few where there was the vein still appeared to be fairly green. I did not see any white, though that should show up later, and even though I did not see leaves curling upward it could still be a Mg. deal in the earlier stages.
If Mn. necrotic and yellow spots will form on top leaves. Mn deficiency occurs when large amounts of Mg are present in the soil.
On top leaves, not on the tops of leaves, so since your effected leaves did not appear to be top/upper leaves I think that can be ruled out … if I am correct in guessing the general effected leaf location.
Now if it is K - POTASSIUM thingy the affected plants are usually the tallest and appear to be most vigorous. Necrotic spots form on lower leaves. Red stems. Leaves appear pale or yellow. A fix is add chemical fertilizer containing K.
Other than just calling what I saw in the pictures necrotic spotting I can’t say I saw any of the rest mentioned, but with them being your plants you might have noticed something the lens did not pick up or maybe you will first see it tomorrow or saw it a few days ago and it did not strike you at the time, anyway if is looks like the above, then you might have hit a bullseye, but I would not hold my breath waiting to be sure.
If it were an Iron problem the leaves on growing shoots would turn pale and veins remain dark green. pH imbalances make iron insoluble. A fix is a foliar feed with chemical fertilizer containing Fe or rusty water. Other than the veins remaining dark it isn’t close so ignore that one.
A Zinc problem would cause white areas first on leaf tips and then between veins. One picture had an odd coloring to it and it almost looked slightly white, but I figured it was just the lighting, but if the brown spots began as white and first at the leaf tips it could be a possibility, but only one of low probability.
Think back over the last few days or a week and see if you can remember a beginning, as in the bottom third of the plant(s) or the middle third, etc. What sort of progression followed? Did it remain more or less on that level or did it spread upwards or did it spread downwards or did it just seem to head off in random directions, both up and down at about the same time? Did you change anything lately, a mixture amount, even just slightly? Do you have a cat and if so could it have sprayed the plant(s), or might you have sprayed the plant(s)? I won't ask the same about a wife or girlfriend or whoever. Just about anything might hold a key bit of information even if it might seem trivial.
That is getting close to being just as far as my brain can take me at the moment those are the most common problems that in any way, even remotely, could be connected to what I could see in the pictures. With luck one of the above might describe something that rings a bell in your head and you say, yep, that’s just how it happened, just like that …. and that one will be your answer …... that is IF it is up there to be found.
