Why is lemon juice added to lemon pie filling after the starch has thickened the water?

Lemon juice is added to lemon pie filling after the starch has thickened the mixture to preserve its bright, fresh flavor and acidity. If lemon juice is added too early, the heat can diminish its tartness and alter its taste. Additionally, adding it at the end allows for better control over the balance of flavors, ensuring the filling maintains the desired tanginess. This technique also helps to prevent any potential curdling of the filling.