Preheat the oven to 325°F. Line a 9x5-inch loaf pan with parchment and grease the exposed sides.
Whisk the almond flour, coconut flour, baking powder, and salt in a bowl.
In a large bowl, whisk the eggs, granulated allulose, Greek yogurt, melted butter, and vanilla extract until smooth.
Stir the dry ingredients into the yogurt mixture until a thick batter forms. In a small bowl, mix the brown sugar substitute, cinnamon, and melted butter for the swirl.
Spread half the batter into the loaf pan, spoon on most of the cinnamon swirl, and add the remaining batter. Spoon the rest of the swirl on top and drag a toothpick through the batter a few times.
Bake for 42 to 50 minutes, until the loaf is set in the center. Cool in the pan for 20 minutes, then transfer to a rack.
Whisk the glaze ingredients together and drizzle over the cooled cake.