Preheat the oven to 375°F and line a baking sheet with parchment paper.
In a large bowl, combine the ground beef, breadcrumbs, egg, diced onion, garlic powder, salt, and black pepper. Mix until just combined, then shape into a loaf about 8 inches long.
Place the loaf on the prepared baking sheet and bake for 35 minutes.
While the meatloaf bakes, combine the root beer, ketchup, brown sugar, mustard seeds, and apple cider vinegar in a small saucepan. Bring to a simmer over medium heat and cook for 8 to 10 minutes, stirring occasionally, until the mixture thickens slightly.
Remove the meatloaf from the oven and brush the glaze evenly over the top and sides. Return to the oven and bake for another 15 minutes, or until the internal temperature reaches 160°F.
Let the meatloaf rest for 5 minutes before slicing and serving.