Turn your Instant Pot on saute. Add the bacon pieces and let them cook until crispy, about 5-8 minutes. Remove with a slotted spoon and set aside. Drain grease from bottom of pot, leaving just a light coating.
Add diced potatoes, chicken broth, onion, garlic, salt, and pepper. Scrape the bottom of the pot with a wooden spoon or spatula to remove all the browned pieces off the bottom. Cook for a minute or two and then tur off Saute Mode.
Lock the lid and turn knob to SEALING. Set to pressure cook on high pressure for 3 minutes. When time is up, quickly release the pressure by turning the knob away from sealing
Open the lid and mash the potatoes until some chunks remain. Add can of evaporated milk, sour cream, and cheddar cheese. Stir well. If the soup is too liquidy, add dried instant potato flakes a few Tablespoons at a time until you reach the desired thickness.
Top bowls with reserved bacon and sour cream, cheddar, scallions, and/or chives before serving.