Prep the oven and pan: Heat the oven to 375°F (190°C).
Line a 12-cup muffin tin with paper liners or grease lightly.
Mix the dry ingredients: In a large bowl, whisk flour, sugar, baking powder, baking soda, salt, and cinnamon until well combined.
Whisk the wet ingredients: In a separate bowl or large measuring cup, whisk eggs, yogurt, milk, oil, and vanilla until smooth.
Combine gently: Pour the wet mixture into the dry ingredients. Stir with a spatula until almost combined. A few dry streaks are fine.
Fold in the rhubarb: Add the diced rhubarb and fold just until distributed. Do not overmix; the batter should be thick and slightly lumpy.
Fill the cups: Divide the batter evenly among the 12 muffin cups, about 3/4 full.
Sprinkle with coarse sugar if using.
Bake: Place on the center rack and bake for 16–20 minutes, or until the tops are set and a toothpick comes out with a few moist crumbs.
Cool: Let muffins rest in the pan for 5 minutes, then transfer to a wire rack to cool. Serve warm or at room temperature.