You may be surprised to discover that they don't really mix well when you pour hot coffee over milk and maple syrup.  As a result, your drink will become lumpy and curdled. This reaction that causes curdling may appear strange, yet there is a scientific justification for it.
Coffee causes milk and maple syrup to curdle due to an interaction between the high temperature of the coffee and the proteins found in milk and maple syrup. When the proteins are subjected to the heat of the coffee, they coagulate, which means that they come together in large clumps. This is what causes the mixture to become lumpy and curdled.