Push saute. Spread butter on all sides of pork. Sprinkle with salt. Add pork to instant pot and sear until browned on all sides, about 3 min per side. Push cancel.
Top with mushrooms, onion, carrots, and ginger.
In a small bowl or shaker cup, combine broth, aminos, miso, and red pepper flakes. Pour over pork. Press cancel.
Lock lid; turn valve to sealing. Program for manual-high and 20 minutes. Allow for natural pressure release. When pin drops, open lid and remove pork to a cutting board. Allow to rest 5 minutes. Shred pork.
While pork is resting, add noodles to instant pot. Lock lid and let stand 5 minutes. Open lid and stir in pork. Top with cilantro and green onions.