Whisk soy sauce, pineapple juice, brown sugar, rice vinegar, garlic, ginger, sesame oil, water, and peppers.
Stir cornstarch with cold water until smooth and lump-free; set the slurry aside.
Heat neutral oil in a large skillet over medium-high heat until shimmering.
Sear chicken in a single layer, stirring, until browned and nearly cooked, about 5–6 minutes.
Add pineapple chunks and cook until lightly caramelized, about 2 minutes.
Pour in the teriyaki mixture and bring to a lively simmer for 2–3 minutes.
Drizzle in the slurry while stirring; simmer until glossy and thick, 1–2 minutes.
Fold in scallions, sprinkle with sesame seeds, and serve hot over steamed rice.