Need some advice, they don't look happy :(

Sounds like the ph is drifting in the medium if your runoff is 5.8. Try adding it in at a higher rate like 6.8 and see how it goes. Some will say this is from nutrients that are not buffered. You want to collect your runoff after the fist bit has pissed out. this will always be off as its a collection of a lot of bits coming out. Also calibrate your PH pen if you have not yet. as they do go a sway and give you a false reading and make you think shits bad. I would also hold onto a bit of your next feeding solution and set it aside and remeasure its PH a every so often or 24-48hrs after the fact and see if the PH is indeed swaying from the nutrients. I do this to find out how much PH down I should be adding into my water. But I feed just water and dry amendments are in the soil. I also find when I grow in peat my PH always drops mid veg/early flower. Its also why I like coir a bit more. Good Luck!

honestly your plants look perfectly fine. it may be more of a cosmetic issue, such as variegated leaves. (the yellow markings)