Pat beef dry with paper towels and season all over with salt and pepper. Heat butter in instant pot using SAUTE function until melted and hot, then add beef roast cubes. Sear beef on all sides, turning occasionally until golden brown. Turn instant pot off and top beef with pineapple tidbits.
In a medium bowl, whisk ketchup, reserved pineapple juice, sugar, Worcestershire, vinegar, and seasonings. Pour over roast and fasten lid shut on instant pot.
Cook beef on high pressure 35 minutes, then let release pressure naturally for 10 minutes. Use two forks to shred beef roast then toss back into sauce. Keep warm in instant pot until ready to serve with buns. Enjoy!