Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with parchment liners or lightly grease with oil or butter.
In a large bowl, whisk together almond flour, coconut flour, cinnamon, apple pie spice, baking powder, baking soda, salt, and sweetener. Break up any clumps so the mixture is even.
In a separate bowl, whisk the eggs, almond milk, melted butter (or oil), vanilla, and apple cider vinegar until smooth and slightly frothy.
Pour the wet mixture into the dry ingredients.
Stir with a spatula until just combined. The batter should be thick but scoopable. If it seems too thick, add 1–2 teaspoons more almond milk.
Fold in the chopped apple and any optional nuts, keeping the mix-ins evenly distributed without overmixing.
Divide the batter evenly among the muffin cups.
If using the cinnamon-sweetener topping, sprinkle it over each muffin.
Bake for 18–22 minutes, or until the tops are set and a toothpick inserted in the center comes out mostly clean with a few moist crumbs.
Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. This helps them set and improves the crumb.