Set the Instant Pot to sauté mode. Season the pork cubes with salt and pepper, then add them to the pot and brown on all sides for about 5 minutes.
Add the diced onion, sliced red bell pepper, and minced garlic. Stir and cook for 1 minute until the onion starts to soften.
Pour in the soy sauce, rice vinegar, brown sugar, chicken broth, and the reserved pineapple juice along with the pineapple chunks. Stir to combine and scrape any bits from the bottom.
Seal the lid, set to high pressure, and cook for 20 minutes. Use quick release once the timer ends.
Mix the cornstarch with 2 tablespoons of water to form a slurry. Switch the pot back to sauté and stir in the slurry until the sauce thickens, about 2 minutes.
Turn off the pot and top the stew with sliced green onions before serving.