I don't really know and Larry will correct me if I'm wrong again but I am thinking the heat is leaving the thin areas too fast and staying in the thicker parts near the mandrel longer.
My guess at a fix would be to take 4 times longer evening out the heat distribution at the back of the flame than you think you need in an attempt to get most of it to an even temperature before putting in the kiln.
Good luck.
Got any pictures? Some times the cracking pattern can give clues at to how the stress is distributed.
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