In your case I think it isnt from the medium. I think it's the humidifier blowing directly at the leaves. They have a shine to them. Which means the will have constant water on them preventing the plants from adequate transpiration. Even if you are watering correctly the plant cant expel the water it absorbs through the medium.. giving it an overwatered look.
But you are correct overwatering and under watering symptoms are similar.
Hope this helps 🙂