I think you really need to send the next one that dies off for a necropsy--if it's IBD, reovirus, or paramyxovirus, or one of the other nasties, they may at least be able to guess at which one, and give some advice. Some of these diseases can be passed between all species of snakes, and even some lizards.

I also recommend finding an alternative to freezing. Snakes do not hibernate--they brumate. That means they stay awake when it gets cold, they don't fall asleep. They just slow down a lot. So, the snake would be conscious as it starts to freeze. It can't be considered humane, or quick, unless you're using liquid nitrogen, and the animal is quite small. It's no longer recommended by any veterinary association, for this reason. There are still a lot of people around who remember when it was recommended, and pass the info on, unfortunately. Due to a reptile's slow metabolism, there are few ways to humanely euthanize one, sadly. If that's your goal...well, there simply is no choice but to stop freezing and use another method. Not condemning you, of course--you only knew what you were told. I recommend looking into it further on veterinary sites and research papers if you want confirmation on this.