Heat oven to 350°F. Grease a 9x5-inch loaf pan and line with a parchment sling.
Stir brown sugar and 1½ tsp cinnamon together in a small bowl for the swirl.
Toss diced apples with lemon juice and set aside.
Whisk flour, granulated sugar, baking powder, salt, and 1 tsp cinnamon in a large bowl.
Whisk melted butter, milk, eggs, and 2 tsp vanilla in a separate bowl until smooth.
Pour wet ingredients into dry and stir just until combined; do not overmix.
Fold half the apples into the batter.
Spread half the batter in the pan. Scatter with half the remaining apples, then half the cinnamon-sugar. Repeat with remaining batter, apples, and cinnamon-sugar. Swirl gently with a knife.
Bake 50–60 minutes, until a toothpick comes out clean. Tent with foil if browning early.
Cool 15 minutes in the pan, then lift out to a rack and cool completely.
Whisk powdered sugar, 2½ tbsp milk, and ½ tsp vanilla. Drizzle over the cooled loaf and let set before slicing.