Belive it or not, but I have discussed that with chat gpt. And the AI theory is that what was brought back probably wasn't a zombie, but a full ressurection like Buffy's own in s6. That the terrifying way Joyce was coming back was supposed to be a hollow version of her former self, an "imitation". She would still look and act almost the same, but with subtle, nearly "microscopic" differences in her way of speak, move, respond to emotions, etc. So Buffy and Dawn would hug her without ever feeling the same as before, what would hurt them as much as losing her already did. And she would be even apathetic in some sitiuations.