Preheat oven to 375°F. Line a 12-cup muffin pan with liners or grease lightly.
Stir crumb flour, both sugars, cinnamon, and salt in a bowl. Drizzle in melted butter and mix until clumps form. Chill.
Whisk 2 cups flour, sugar, baking powder, baking soda, and salt in a large bowl.
Whisk buttermilk, oil, eggs, and vanilla in a separate bowl until smooth.
Toss chopped rhubarb with 1 tbsp flour to lightly coat and help suspend in batter.
Pour wet ingredients into dry. Fold gently until a few streaks remain. Fold in rhubarb just to combine.
Scoop batter into muffin cups, about 3/4 full. Sprinkle generously with crumb topping, pressing lightly to adhere.
Bake 18–22 minutes, until the tops are set and a tester comes out clean or with a few crumbs.
Cool 5 minutes in pan, then transfer muffins to a rack to finish cooling. Serve warm or at room temperature.