Turn on a multi-functional pressure cooker (such as Instant Pot) and select Sauté function. Add oil and let heat for 1 minute. Add bacon and cook until bacon is crispy and fat has rendered out, about 8 minutes. Transfer bacon to a paper towel-lined plate using a slotted spoon.

Add onion and celery to bacon drippings and cook for 2 minutes. Add garlic and thyme and cook for 1 minute. Cancel Sauté mode. Add potatoes and broth. Close and lock the lid. Select high pressure according to manufacturer’s instructions; set timer for 8 minutes. Allow 10 minutes for pressure to build.

Release pressure using the natural-release method according to manufacturer’s instructions, about 10 minutes.

Whisk together 2 tablespoons milk and cornstarch in a small bowl to make a slurry. Open the pot and stir in cornstarch slurry and 1 cup milk. Season with salt and pepper. Let soup sit, stirring several times until thickened, about 10 minutes. Ladle into bowls and sprinkle with bacon.