If the witches in the field defending against the Red Dragon had the philosopher's stone, which caused their mana flow to stop after a fair amount of time, then why did the witch in the castle who also eat it have her mana flow stop immediately ?
Bu, MC's lich, seemed to absorb the witch's philosopher's stone after she died - thus it did not look like Bu affected the witches defending in the field.
Maybe a novel reader can clarify.