Preheat oven to 350 F and line a 12-cup muffin tin with paper liners. In large bowl, whisk together flour, beet powder, oats, baking powder, baking soda, cinnamon, and protein powder. In another bowl, mix milk, maple syrup, coconut oil, eggs, and vanilla extract. Gradually add wet ingredients to dry ingredients, stirring until just combined. Gently fold in mixed berries. Divide batter evenly among muffin cups and bake for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean. Allow muffins to cool before serving.