Open the pineapple can and reserve 1/4 cup of the juice; leave the pineapple chunks in the can with the remaining juice to add to the Instant Pot.
Add the chicken pieces, low-sodium soy sauce, sesame oil, minced garlic, grated ginger, hoisin sauce, brown sugar, red pepper flakes (if using), and the pineapple chunks with their juice (not the reserved 1/4 cup) to the Instant Pot; stir to combine.
Close the lid and ensure the pressure valve is sealed. Cook on Poultry or Manual at High Pressure for 4 minutes.
Allow a 5-minute natural pressure release, then carefully release any remaining pressure and open the lid.
Select the Sauté function on LOW. In a small bowl whisk 1 1/2 tablespoons cornstarch with 3 tablespoons of the reserved pineapple juice until smooth and lump-free.
Stir the cornstarch slurry into the Instant Pot and cook on Sauté, stirring gently for a few minutes until the sauce thickens; if a thicker sauce is desired, mix 1 additional tablespoon cornstarch with 1 tablespoon juice and stir in.
Let the chicken rest 5–7 minutes; the sauce will thicken further.
Serve the pineapple chicken over rice and garnish with sliced green onions, sesame seeds, and slivered almonds.