In Instant Pot using sauté function, cook pancetta in oil, uncovered, for 5 to 7 minutes, or until fat has rendered. Transfer pancetta to plate.
Meanwhile, pat chicken dry with paper towels; season all over with thyme and 1/2 teaspoon each salt and pepper.
In batches, add chicken, skin side down, to pot; cook 6 minutes or until browned on two sides, turning once halfway through. Transfer chicken to large plate.
To pot, add leek and 1/4 teaspoon salt; cook 3 minutes, stirring. Add wine. Hit cancel, reselect sauté function, and adjust heat to More; heat to boiling. Hit cancel again, then reselect sauté function (this returns heat to Normal); simmer 5 minutes. Hit cancel to turn off sauté function.
Add potatoes, mushrooms, and chicken to pot. Cover and lock lid. Select Manual/Pressure Cook and cook at high pressure for 8 minutes. Once cooking is complete, release pressure by using a quick release.
Serve chicken and vegetables with some cooking liquid. Garnish with parsley.