Why does non-alcoholic apple cider mix smoothly with milk, but mixing apple juice with milk causes it to curdle?
Apple juice contains organic acids which causes the proteins present in milk to coagulate (curdle). The non-alcoholic cider most likely (depends on the brand?) contains less acids, so there is no coagulation. Good thing you mentioned that there is no alcohol present in the cider - ethanol is another thing that can cause milk to curdle.