I have grown a number of fargesia species, including two species from seed, although I have not grown murielae. However, I would not expect murielae to behave much differently from other fargesias. My experience with fargesia seedlings is that they can be slow to start off, but what you are experiencing seems to be approaching abnormal.

Has this clump put up any new, small culms since you got it? Is it out of the pot and in the ground, or at least in a bigger pot than the original? And how certain are you that this plant was properly identified by the seller?

Definitely bigger than the original, perhaps 2 or 3 times the small culms I started with. It stays evergreen in the winter and I don't seem to be losing culms to winter kill. Just that none of the new ones are any taller than 12".

I suggest moving it to a more protected area. Winter temps and winds could be setting it back each year. Mulch it heavily for a few years and see how it goes. My rufa is very slow it has only grown about 4 inches taller in 3 years. the plant is beautiful but I'm afraid I'll be dead before it matures.