Just an opinion, looks like the beginnings of Magnesium (Mg) deficiency possibly.
Check your ph. Water in, Water out, and coco/soil if possible. 6 to 7.
You can safely try a foliar spray of 1/4 tsp epsom salts per gallon of water. Mix well. Mist both top and underside of leaves. Repeat in 5-7 days if you notice improvement.
If it helps, you can continue with foliar, or it can be added to your usual fertilizer/watering schedule once a week or so.
I had Mg deficiency so often, the addition of epsom salts became standard. This for both peat and coir mixes.
Hopeful some others will chime in and offer their opinions.
Good luck.