Preheat oven to 425 degrees F. Line a muffin pan with muffin liners. Set aside. (Recipe makes 12 heaping muffins.)
Whisk together the dry ingredients - flour, poppy seeds, pumpkin spice, baking powder, baking soda, and salt - in a medium bowl.
In a large mixing bowl, combine sugar and orange zest. Rub together to flavor the sugar.
In a large measuring cup or bowl, combine liquid ingredients - greek yogurt, milk, melted butter, eggs, orange juice, and vanilla extract.
Pour half the dry ingredients and half the liquid ingredients into the bowl with the orange sugar. Stir to combine.
Pour the other half of dry and liquid ingredients into the mixture, stir again just until combined. Don’t overmix.
Scoop muffin batter into the prepared muffin pan. Divide evenly between 12 muffins.
Make the cinnamon crumb topping: combine brown sugar, flour, oats, cinnamon, and cold butter in a mixing bowl. Use a pastry cutter, knife, fork, or food processor to cut the butter into the other ingredients until it forms medium-sized crumbs.
Evenly top all the muffins with the crumble.
Bake at 425 degrees F for 8 minutes. Without opening the oven, reduce the oven temperature to 350 degrees F and bake another 7-10 minutes, or until a toothpick inserted in the middle comes out clean.
Remove muffins from the oven and allow to cool for 10 minutes before removing and placing them on a cooling rack to cool further.
Meanwhile, make the maple glaze: in a small bowl, combine powdered sugar, maple syrup, and ½ tablespoon of water. Stir to combine. Add more water if needed to help thin out the glaze. Drizzle over the top of the cooled muffins.