i thought the same as well, but i think the ph of 5.8 is what caused the issue. had them under really intense lighting and 5.8 is a tad out of the cal/mag uptake range for coco and i think they were just growing so damn fast they werent able to keep up with the ph being low.
now im running 6.3 with a ppm of 600 base and 150 ppm of calmag on top of that for a total of 750.
been hitting them with soil balance pro beneficials, kelp and molasses thats been brewing for a few days hand watered on top of their regular food once a week.
seeing lots of HEALTHY root growth so thats a good sign i think.
I'm having salt buildup too. Ive been flushing every other feed for now growth has resumed. I'm feeding at .8-.9 ec jack's and I flush with PhD tap. I have no choice right now to feed but once a day and I went straight from rooted clones to 4g pots so I need the dry cycles to build up the roots.
On the plus side I finally got my drip system installed so I may start feeding twice a day.
Damage to your plants will show very fast when coco dries out as the PH swings way out of the useful range. If your pots are drying out quick like that, you are a candidate for multi waterings per day and that is part of what is so magical about coco. You will see good stuff happen if you water at least twice a day. I used to water every two hours on a timer and while that was a little much, it worked just fine in coco.