Place chicken breasts in a multi-functional electric pressure cooker (such as Instant Pot(R)) and add 2 cups chicken broth, cream of chicken soup, milk, butter, ranch dressing mix, and onion powder. Close and lock the lid. Select the Poultry setting according to manufacturer's instructions; set timer for 25 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid.
Transfer chicken to a cutting board using a slotted spoon. Shred chicken and return to the pot. Add rotini, remaining chicken broth, and bacon. Close and lock the lid. Select the high pressure setting according to manufacturer's instructions; set timer for 5 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid.
Stir in shredded mild Cheddar cheese and cheese sauce until well combined.
