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This seems like a relevant thread to ask:
Does anyone know if the shininess of an egg is determined at the egg's creation or at the time that the pokemon hatches? I'm considering filling up Isle Avue with eggs and soft resetting for shinies but I feel like I might just be wasting time.
I guess I could do a trial run and see if the natures and stats are different after an SR, I suppose that'd be a good indication that the pokemon's attributes aren't determined until it hatches.