Oh, I've an idea.The negative energy which is used to create undead is said to give them some perverse imitation of life.
Then we should leave our modern understanding of how poisons and diseases work (which is really easy to a lot of people given recent events), and they obviously don't work like that in Golarion anyway.
Now it's rather simple to imagine that poisons and diseases interact with that imitation of life in a similar way. Poisons and diseases aren't positive or negative, they can't really distinguish between them. But because that is an imitation, and indeed twisted, they can't always have the same effect. Most of the time they don't work at all. But for some creatures like PCs these ailments still function. PCs have also other differences like more power, free will and a chance to be non-evil. Guess they are really unique.
So this has to be my favourite answer in-world. Perhaps PCs retain more of their practical biology and spiritual self-ness simply due to more exposure (and thus adaptation) to varied magical radiation over the years? Sure, your classic Lich has probably seen some weird spells in their quest for immortality, but nothing matches the onslaught of background level magic nonsense that PCs go through on their way to breakfast.
Or you are one of the very small contingent used in Urgathoa's experiments for a new kind of undead.
You are the Chosen one.
That said, this plot hook. The Chosen One and you certainly don't want to be. Especially if she's sending anyone to collect the results...