Check the roots. Take a photo of them. Post the pic.
I suspect root damage.
Pleated leaves doesn't necessarily mean they're not getting enough water. It could also mean that a lot of the roots got damaged, and the plant is not getting enough water to grow right.
Always check the roots.
Also be aware of the pot size.
I recommend using a clear plastic pot so you can see what's going on with the roots. The roots are photosynthetic, btw, they can use light to supply more energy for the plant.
I also don't know what you mean by "finer commercial mix". Please post a photo of your plant in the mix.
Miltoniopsis are epiphytes of mid to high elevation forests that stay consistently moist year round. The branches they grow on are almost always covered in mats of lush growths of living moss.
https://www.flickr.com/photos/andreaskay/8563806741/
The roots do like good air circulation too. Make sure the pot's got quite a bit of drain holes or slots. Water shouldn't sit in there at all.
A humidity of 50% is ok for them, they don't need to be at 90% the whole time.
If it dries out within 1 day, water twice a day. Do not be lazy and sit the pot in standing water. That is a pretty quick death sentence for these guys.