In a medium bowl, whisk together the soy sauce, gochujang, beef broth, brown sugar, if using, garlic, ginger, tomato paste, sesame oil and red pepper flakes. Set aside.
Season the short ribs all over with salt and pepper. Turn the Instant Pot sauté setting to high. When hot, pour in 2 Tbs. of the oil. When the oil is hot, add about 4 to 5 short ribs to the pot. Sear on all sides, about 2 to 3 minutes per side. Place the browned short ribs on a plate. Repeat with the remaining short ribs.
Return all the short ribs to the instant pot. Whisk the sauce, then pour it over the ribs. Secure the lid, and set the pressure valve to seal. Pressure-cook on high for 45 minutes, followed by naturally releasing the steam for 15 to 20 minutes. Release the remaining pressure by adjusting the pressure valve to the venting position. Open the lid.
Remove the short ribs to a bowl. Pour the sauce into a saucepan and skim off the fat. In a small bowl, whisk together cornstarch and ¼ cup water. Whisk the mixture into the sauce and simmer until thickened slightly. Stir in the rice vinegar.
Serve the ribs sprinkled with scallions and sesame seeds, and the sauce poured over.