Combine cumin, chili powder, oregano, salt, and black pepper in a bowl. Sprinkle over both sides of the chicken breasts.

Turn on the Instant Pot and select the Sauté function. Add 1 tablespoon olive oil, and sear chicken breasts in batches for 1 to 2 minutes per side. Transfer chicken to a plate.

Add onion and garlic to the Instant Pot. Cook and stir until translucent and golden, about 2 minutes. Return chicken to the pot; add cilantro, lime juice, chicken broth, and other aromatics. Close and lock the lid. Select high pressure and set the timer for 8-10 minutes. Allow pressure to build for 10-15 minutes.

Combine mayonnaise, chipotle pepper, milk, salt, and garlic powder in a blender. Blend until smooth. Refrigerate sauce until ready to serve.

Release pressure using the natural-release method, which takes 10-40 minutes. Unlock and remove the lid.

Preheat the oven's broiler and prepare a baking sheet with olive oil.

Shred the cooked chicken and coat with reserved cooking liquid. Spread chicken on the baking sheet and broil for 5 minutes. Rotate the sheet, toss the chicken with a little more liquid, and broil for another 5-7 minutes.