Combine soy sauce, chicken broth, pineapple juice, brown sugar, sherry, honey, garlic, sesame oil, ginger, and red pepper flakes in a saucepan over medium heat. Whisk the cold water and cornstarch in a small bowl until you have a thin slurry, and add to the saucepan, stirring to incorporate.
Bring sauce to a boil. Reduce heat and simmer to slightly thicken, stirring occasionally, 3 to 4 minutes. Remove teriyaki sauce from heat.
Place pork roast in a slow cooker and pour teriyaki sauce over the top. Cook on Low until the pork is tender and falls apart when pulled with a fork, 6 to 8 hours.
Remove the pork from the cooker and let rest for 5 minutes before shredding with 2 forks.
Return the shredded meat to the pot, mix with the sauce, and let it warm up for about 5 minutes. Sprinkle with sesame seeds and serve.