Select the sauté setting and adjust it to high (more). Add the oil to the pot. When it shimmers, add the onion, celery, bell pepper, and garlic. Cook, stirring occasionally, until the vegetables start to soften, about 3 minutes. Add the beef and cook, breaking up the meat into small pieces with a wooden spoon, until no longer pink, about 4 minutes. Add the tomato sauce, ketchup, water, Worcestershire sauce, vinegar, brown sugar, chili powder, salt and pepper. Stir to combine.
Lock the lid into place and turn the valve to “sealing.” Select pressure cook (manual) and program for 10 minutes at high pressure. When the cook cycle completes, allow the pressure cooker to cool without venting steam for 5 minutes. It's fine if the keep warm setting turns on. After 5 minutes, turn the pressure release valve to “venting” to manually release the steam. When the pressure valve drops, carefully remove the lid. (Always remove the lid away from your face.)
Stir the Sloppy Joe mixture. You can serve as is or you can thicken the sauce. To thicken, stir together the cornstarch and warm water in a small bowl until smooth. Add the cornstarch mixture to the beef mixture. Set the pot to “sauté” (You might have to turn off your pressure cooker and turn it back on to do this.) Cook, stirring occasionally, until the mixture bubbles and thickens, about 2 minutes. Taste. Season with salt and pepper as needed.
Spoon Sloppy Joe mixture on the bottom halves of the buns. Cover with the bun tops and serve immediately.