Pat the chicken thighs dry with paper towels and season both sides with salt and black pepper.
In a medium bowl, whisk together the pineapple juice, soy sauce, brown sugar, rice vinegar, garlic, and ginger.
Place the chicken in the Instant Pot and pour the pineapple juice over the top. Lock the lid in place and cook at high pressure for 10 minutes. Allow the pressure to release naturally for 10 minutes, then carefully release any remaining pressure.
Transfer the chicken to a plate or large bowl. Shred it into bite-sized pieces with two forks.
Select the sauté function on the Instant Pot. Whisk together the cornstarch and cold water until smooth. Stir the slurry into the sauce and simmer for 2 to 3 minutes, or until the liquid becomes glossy and slightly thickened. Stir in the sesame oil, pineapple chunks, and shredded chicken. Cook for another minute so the chicken is thoroughly coated and the pineapple is warmed through.
Warm the tortillas in a skillet or microwave until flexible. Fill each tortilla with the saucy Hawaiian chicken. Top with pineapple slaw, onions, cilantro, or sliced jalapeños if desired.