probably no eggs, which is what holds the oil and water together

Mayonnaise Style Sauce ingredients: Soybean oil, Water, Liquid egg yolk, Vinegar, Sugar, Salt, Mustard seeds, Mustard bran, Xanthan gum, Potassium sorbate, Calcium disodium EDTA.

Miracle Whip ingredients: water, soybean oil, sugar, vinegar, modified cornstarch, egg yolks, salt, mustard, spices, potassium sorbate, calcium disodium EDTA, dried garlic.

Thankfully you don't have to be too concerned. It's for the same reason Miracle Whip doesn't actually call itself mayonnaise, and Hellmann's Real Mayonnaise does. Mustard, garlic, etc., are not mayonnaise ingredients.

mustard is definitely a mayonnaise ingredient

Indeed, you're right. I thought at its most basic it is just egg whites and oil, but googling that specifically says mustard is part of the recipe.

So it's still a mystery why McDonalds doesn't just call it Mayonnaise when the ingredients are pretty much identical to every other mayonnaise out there, including Hellmann's REAL Mayonnaise.

Maybe it's one of those "It's only X if it comes from the X region" type issues. It just makes it sound suspicious for no discernable reason. Like buying a bottle of "Wet 'water' style beverage."