My intuition says its a funny oil that caused the problem. Eggs are eggs. Mustard is mustard. But the oil is the variation from normal. When I make mayo with peanut oil, eggs, mustard and lemon it is fridge-stable until I use it up weeks later. Whether it is an impurity in your oil, or the oil itself...who knows? Why not do an experiment. Make mayo with avocado oil and with veg oil. See which lasts longer.