I don’t know the answer either, I was taught to use 316 but never knew why. Here’s an interesting article about the properties of different stainless welding rods ~ doesn’t directly answer your question, but explains why 316 is probably recommended (corrosion resistance).
http://www.weldingtipsandtricks.com/...allurgy-2.html
Do you think you might have gotten some mislabeled rod? I don’t recall having a similar problem, but I haven’t made new mandrels in a very long time....