Cut the chicken thighs into roughly 1-inch pieces and pat them dry. Season with the salt, pepper, and garlic powder. Dip each piece in the beaten eggs, then toss in the cornstarch until fully coated. Shake off any excess cornstarch.
Preheat the air fryer to 400 degrees F. Arrange the coated chicken pieces in a single layer in the basket, leaving space between them for air circulation. Spray the tops generously with cooking spray.
Cook for 10 minutes, then flip each piece and spray again lightly with cooking spray. Cook for another 5 to 6 minutes until the chicken is golden, crispy on the outside, and cooked through.
While the chicken cooks, prepare the sweet and sour sauce. In a small saucepan over medium heat, whisk together the ketchup, rice vinegar, brown sugar, soy sauce, pineapple juice, and cornstarch. Bring to a simmer, stirring constantly.
Cook for 2 to 3 minutes until the sauce thickens and turns glossy. Remove from heat and stir in the sesame oil.
Transfer the crispy chicken to a large bowl. Pour the warm sweet and sour sauce over the chicken and toss gently until every piece is coated.
Serve immediately over steamed rice, garnished with sesame seeds and sliced green onions. Serve quickly so the chicken stays crispy on the outside while glazed with sauce.