Open the pineapple can and reserve 1/4 cup of the juice; leave the pineapple chunks and remaining juice in the can to add to the pot.
Add the chicken pieces, soy sauce, sesame oil, minced garlic, grated ginger, hoisin sauce, red pepper flakes (if using), brown sugar, and the pineapple chunks with their remaining juice to the Instant Pot; stir to combine.
Close and seal the lid. Set the Instant Pot to Poultry or Manual on High Pressure for 4 minutes.
When cooking finishes, allow a 5-minute natural pressure release, then carefully quick-release any remaining pressure and open the lid.
Turn the Instant Pot to Sauté 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.
Pour the cornstarch slurry into the pot and gently stir. Simmer on Sauté, stirring occasionally, until the sauce thickens, a few minutes.
If a thicker sauce is desired, mix 1 tablespoon cornstarch with 1 tablespoon reserved juice, add to the pot, and cook until thickened.
Let the chicken stand 5–7 minutes to allow the sauce to set. Serve over rice and garnish with sliced green onions, sesame seeds, and slivered almonds.