If all the springs were original, that increases the chances that they were from the same heat treat lot. So if one was improperly heat treated all in the lot would be too. Usually there would be a certification with a "heat number" which identifies the parts heat treated at the same time. The clutch manufacturer might have bought the springs from a wholesaler or a vendor and might not ever have tested them. In high quality manufacturing the springs would have been tested for hardness, and spring lots would not be mixed.
I am not saying this is absolutely the problem, but, there is a high probability.