In a large mixing bowl, combine the shredded chicken, BBQ sauce, pineapple chunks, red onion, mayonnaise, apple cider vinegar, garlic powder, salt, and black pepper. Mix well until all ingredients are evenly coated.
Heat the mixture in a saucepan over medium heat for about 5-7 minutes, stirring occasionally, until warmed through. Alternatively, you can microwave it in a covered bowl for 2-3 minutes.
While the chicken mixture is heating, toast the hamburger buns on a skillet or in a toaster until golden brown.
Once the chicken mixture is heated, spoon a generous amount onto the bottom half of each toasted bun. If desired, add a handful of coleslaw mix on top for extra crunch.
Place the top half of the bun on each sandwich and garnish with fresh cilantro if using. Serve immediately.