It could be several different possibilities.
It could be just normal shedding of the leaf as it ages, or it could be an indication of a problem with the plant.
Can you post a picture of the plant?
I do have seedling dendrobiums in sphagnum moss when they are starting out, but as they age I slowly transition them to bark or open baskets.
So I dont think that the sphag itself would be a problem unless it stays constantly wet, or is packed too tight.