Preheat the air fryer to 350 degrees F. Place the pork along with the salt and pepper in a medium-sized bowl, and mix to combine. Add the cornstarch and toss the pieces until coated. Take the pieces out onto a plate, shaking off any excess cornstarch, and set them aside.
Spray the air fryer's basket with cooking spray, add the onion, bell peppers, and pineapple to the basket, spray the onions, onions, and peppers with cooking spray, and shake the basket to coat. Air Fry for 20 minutes, shaking the basket every 5 minutes, spraying with more cooking spray if it looks like it's drying out. Air Fry until the vegetables are softened and begin to char slightly.
Once they are done cooking, place everything onto a plate, and set it aside.
Preheat the air fryer to 400 degrees F. Spray the air fryer with cooking spray. Place the pieces of the pork into the basket, not touching. You may need to do this in batches. Spray well with cooking spray and air fry for 5 minutes, flipping the pieces at the 3-minute mark.
While the pork is cooking, make the sauce. In a large skillet over medium heat, add the water, sugar, vinegar, and ketchup, and whisk to combine fully. In a small bowl, whisk together the cornstarch and water. Once the sauce is simmering, slowly wish in the cornstarch slurry. Let come back to a simmer to thicken.
Immediately add the cooked pork, onions, peppers, and pineapple to the skillet and toss with the sauce. Serve with a garnish of thinly sliced scallions.