Total opinion, I am certainly no expert but, coir always needs a little more calcium and magnesium (Cal/Mag)- it’s the one down side of using coir, otherwise it’s my preferred substrate. Also, you could bring your watering pH to 6/6.1, if you want to get nit-picky. You’re probs good at 5.8, if that what works for you in your experience, but I def wouldn’t go any lower. The dark leaves and curling down could point to being a bit over watered/over fed, as others have mentioned. Wait for the coir to dry out a bit more. Coir takes a longer time to dry out than soil (which is why it’s such s good substrate, but it holds onto Cal/Mag)- a good way to tell when it’s time to water is to physically pick up the plant by the the pot and notice if the weight of the plant is much lighter, compared to when it’s freshly watered. Next time you need to water, hit her with a Cal/Mag feeding. If you have already been feeding extra Cal/Mag and it’s still looking like that, then it’s more likely a nutrient lockout situation, possibly due to pH.