That plant seems plain dried out in my opinion, not thriving. As you said, the medium is inert, but it seems that no nutes are getting absorbed or processed. So if the medium drys out too quickly, yes. I am not familiar with growing in your medium, I cant make sense of it, I know that people on here do it though I believe there is perhaps more moisture delivery involved, in general, in those set ups. I quickly skimmed but you seem to be saying that you're doing everything perfectly, so if that's true it could just be the plant, something to compare it to would be nice right?
Honestly man, and I grow in soils, so everyone can please feel free to call bullshit or whatever but I would see if you could start mixing some vermiculite down into your mix with your fingers or a pencil or something, short of disturbing what growth you may have there. Ive mixed coco in some soil mixes before but it cant be relied upon to retain any moisture, so you need to so something to make sure those roots have access to water, at least more than you seem to be watering it.
If water runs across your tongue once every few days, it might keep you alive, but its not going to quench your thirst, and you will never thrive.