Preheat oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners.
In a large bowl, whisk together the whole wheat flour, baking powder, baking soda, 1 teaspoon cinnamon, and salt.
In a separate medium bowl, combine the applesauce, maple syrup, egg, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined, being careful not to overmix.
Gently fold in the diced apples.
Prepare the crumble by mixing the rolled oats, melted coconut oil, 0.5 teaspoon cinnamon, and 1 tablespoon flour in a small bowl until clumps form.
Distribute the batter evenly among the muffin cups.
Sprinkle the oat crumble topping over each muffin.
Bake for 18 to 22 minutes or until a toothpick inserted into the center comes out clean.
Cool in the pan for 5 minutes before transferring to a wire rack.