[ericthered]

As a player or GM, I'd be fine with the enchanter either drops over dead until the stone is no longer in the zone, or if the caster became mortal for the duration. Either interpretation seems to be a reasonable effect for the stone no longer being active. I'd expect the "correct" answer to depend on setting fluff about souls, and thematically dropping dead makes more sense, and is more dramatic.

I'd have a problem if anti-magic zones just killed the enchanter outright, that feels less resilient than other magic items, and soul stone is all about resilience.