Cut the chicken breasts into 1-inch bite-sized chunks. In a small bowl, combine the garlic powder, salt, and pepper, then toss the chicken pieces with this seasoning mixture until evenly coated. Set the seasoned chicken aside while you prepare the sauce base.
In a medium bowl, whisk together the coconut milk, pineapple juice, soy sauce, brown sugar, minced garlic, ginger, and red pepper flakes until the sugar dissolves and the mixture is smooth. Reserve about 1/4 cup of this sauce in a separate small bowl and set it aside—you'll use this later to create the thickening slurry. Pour the remaining sauce over the seasoned chicken and let it marinate for at least 15-20 minutes (you don't need the full 30 minutes if you're ready to cook—the chicken will absorb flavor as it cooks).
Heat the oil in a large skillet over medium-high heat until it shimmers. Working in batches to avoid crowding the pan, add the marinated chicken (drain most of the sauce from the chicken as you add it) and sear for 3-4 minutes per side until golden brown and cooked through, about 8-10 minutes total. The chicken should be mostly cooked at this point. I like to reserve any accumulated liquid in the pan—it adds extra flavor to your sauce.
Add the frozen stir-fry vegetables directly to the pan with the cooked chicken (no need to thaw them) and stir to combine. Cook for 2-3 minutes, then add the pineapple chunks and the reserved marinade from Step 2. Stir everything together and let it simmer for 2-3 minutes until the vegetables are heated through and the pineapple is warm.
In the small bowl with the reserved sauce from Step 2, whisk in the cornstarch until it's completely dissolved with no lumps. Pour this slurry into the pan with the chicken and vegetables, stirring constantly. The sauce will thicken within 1-2 minutes as it simmers—you'll see it transform from thin and glossy to a light, coating consistency. Taste the dish and adjust the seasoning with additional salt or red pepper flakes if needed.
Transfer the pineapple coconut chicken to serving bowls over cooked rice. The sauce should coat the chicken and vegetables beautifully, creating a creamy, tropical dish with just the right amount of thickness.