Crisp the pancetta. Turn your Instant Pot onto the 'sauté' function. Add the diced pancetta and cook for 4-5 minutes, stirring regularly until the pancetta has crisped up and rendered some of its fat. Transfer pancetta to a plate and set aside.
Brown the beef. Add the olive oil to the Instant Pot and the cubes of beef into a single layer. Let the meat get very brown on all sides. You may have to do this in two batches. Remove the beef from the pan and set aside with the pancetta.
Sauté the aromatics. Add the onions, carrots and celery and cook for 4-5 minutes, stirring occasionally until soft. Add the minced garlic and cook 1 minute more.
Pressure cook on high. Return the beef and pancetta to the pot. Add the wine, and bring to a simmer. Simmer until the wine is almost completely evaporated and then add the tomatoes and juice, bay leaf, oregano, red pepper flakes, salt, pepper, and milk. Place the lid on the pot and lock into place. Cancel the sauté setting and change to 'meat/stew' high pressure for 1 hour and 15 minutes. Shred the beef, then simmer to thicken. Manually release the pressure on the Instant Pot (make sure to do this away from walls or shelves that could be damaged by steam). Remove the lid, shred the meat with a fork, and transfer again to the sauté function and simmer for 15 min or so until thickened, stirring once in a while so that the ragu doesn't stick.
Meanwhile, cook the pasta. While the ragu simmers, cook your pappardelle until al dente and drain.
Plate and serve. Transfer the pasta to your bowls, top with ragu, mascarpone, freshly grated parmesan, and red pepper flakes (if desired).