Or perhaps the spontaneous mutation is in your male, rather than in his pastel offspring?
It would explain how the original breeder thought there was no possibility the snake carried a pastel gene. And even if you assume the breeder got confused about which animals he'd bred, did he even have a pastel caramel or pastel het caramel to have put in the mix to get the pastel caramel outcome?
Anyway, glad to see the mystery has been at least partially solved! Congrats!