Preheat oven to 350°F (175°C). Grease a 9×5-inch loaf pan and line with parchment if desired.
Mix apples with brown sugar, cinnamon, and 1 tablespoon flour. Set aside.
Mix cinnamon swirl ingredients in a small bowl.
Whisk flour, baking powder, salt, and cinnamon in a medium bowl.
In a large bowl, whisk sugar and eggs until smooth. Add milk and vanilla, then whisk in melted butter.
Add dry ingredients to wet ingredients and stir just until combined.
Layer ⅓ batter into pan. Top with ⅓ apples and ⅓ swirl. Repeat twice, finishing with apples and swirl on top.
Swirl gently with a knife a few times.
Bake 55–65 minutes. Tent with foil if browning too quickly.
Cool 15 minutes in pan, then transfer to a rack.
Whisk glaze and drizzle over cooled (or slightly warm) loaf. Slice and serve.