Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Add onion and butter to the hot pot. Cook and stir until fragrant and onion has softened, about 3 minutes, adding garlic during the last 30 seconds. Remove onion mixture to a small bowl. Hit Cancel. Add chicken broth, chicken breast, and rice.

Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 10 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. Remove chicken breast, and shred.

Add tomatoes and corn to the pot, along with shredded chicken. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 3 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 reserved onion mixture, taco seasoning, cumin, salt, and pepper. Add Cheddar cheese and stir until melted.