I've been battling the same problem for months. I used to have amazing roots in coco. Until I started getting problems.
It's the root infection each and every time. Pythium oospores are in your environment. By letting the pots dry out a bit you are keeping it at bay. But as soon as you saturate the pots their numbers increase and they damage the roots micro hairs and thinner roots. Plant cannot absorb nutrients effectively and show deficiencies.
Until you get rid of all the equipment or sterilize absolutely every thing and every inch of the space and corner you will have this recurring problem. Additionally fungus gnats can transfer it form plant to plant.
People that can get away with keeping the coco saturated all time are lucky to have no pathogens in their environment.
Since I moved to a place near a park with lots of vegetation and bugs I ve been battling it for months. No matter what I ve tried, different coco brands, nutrients, beneficial bacteria, trichoderma, fungicides, h2o2, hypochlorous acid. As soon as I started keeping the coco saturated plants where getting stunted because of rapidly spreading pythium or phytoflora or some kind of root disease. I'm planning to get rid of everyrthing and move to a new place.
Cheers.