I don't think twisting is normal, but some arching of the soundboard is.I have been told that structurally harps are little bit like ships, they must be able to move at least 7 different directions at once, and that there is 100 pounds of torque/stress on the harp per string. A friend who does a little luthier work says that there is no logical reason ANY harp should hold together, that they are not really a musical instrument, but something just designed to make luthiers and harpers more crazy!