The bag may look the same but the contents can vary from batch to batch. All that's guaranteed is the AVERAGE nutritional profile. All sort of different ingredients can go in to reach that, and it's still an AVERAGE, so it will be not enough for half the birds and too much for the other half, since each bird has an individual and different metabolism, and none are usually exactly average - like people, How many of us are average height, average weight, average build?
So maybe the current bag of feed is seriously deficient in one or more respects - assuming that cannibalism is the cause of course.
Old animals need more protein; that is true of people and chickens, and other species.
I cannot comment on guineas, about which I know nothing.