Great question — answering as the UphillTech team:
The spoilage is not about losing protein content but the same kind of spoilage that any food product will undergo. In this case when you mix the protein powder with water you are doing something similar to making milk from milk powder, and as the guidance from the manufacturer says you should use it up promptly. There is nothing about mixing it with oats (which just soaks up some of the water in the mixture) that will make it last longer or shorter, so I would apply the same two-day limit to your overnight oats if you make them with protein powder. If you are committed to making your oats further in advance, make them with water or milk or whatever and then just add the protein powder...
Full answer by dbmag9: https://cooking.stackexchange.com/a/119901 (CC BY-SA 4.0).
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