Place the cut chicken pieces in the slow cooker.
In a mixing bowl, whisk together 1/2 cup chicken broth, light brown sugar, granulated sugar, white vinegar, lemon juice, soy sauce, tomato paste, garlic powder, ground ginger, black pepper, and Sriracha (if using) until smooth.
Pour the sauce over the chicken and press the pieces so they are coated and submerged in the sauce.
Cover and cook on low for 5 to 6 hours, until the chicken is tender and cooked through.
About 20–30 minutes before serving, whisk 1/3 cup chicken broth with 1/3 cup cornstarch in a small bowl until smooth.
Gently fold the cornstarch slurry into the slow cooker along with the diced pineapple, onion, and bell pepper.
Cover and cook on low for another 20–30 minutes, until the sauce thickens and the pineapple and vegetables are slightly tender.
Serve warm over white or brown rice.