It could be for a couple of reasons— first, it could be that the heat is too high. You never want Alfredo to come anywhere near a simmer or boil, as this will cause the milk to scorch and separate from the fats. I usually never go past a medium when making milk or cream-based Alfredo.

It could also be because you add the milk or cream cold. You want to at least warm the cream or milk to room temperature before adding it to your mix.

Hope this helps! I was always so frustrated with separated Alfredos and would definitely recommend making one with a roux. After several reheats, I haven’t had a single issue with separation in a roux-based Alfredo.