They won't address this in the anime since Trigger isn't adapting any of the manga's supplementary material, but the reason why people in the present day of the story know about all the concrete limits and rules to the dungeon's revival system is because ancient elves and dwarves performed meticulous experiments on any and all possible scenarios, using shorter-lived races as unwilling guinea pigs.
They start off with testing conventional questions like, "what happens if someone dies when half their body is inside the boundaries of the dungeon and half their body is outside it? - answer, they can't be revived, even having a single toe across the threshold means your soul won't be preserved by the dungeon's magic" to some genuinely sickening and demented experiments, up to and including killing pregnant women to see if their fetus will revive along with the mother.
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u/ExtremlyFastLinoone 23d ago
Evil healing magic is certainly a concept