Turn on a multi-functional pressure cooker (such as Instant Pot(R)) and select Saute function. Melt butter. Add sweet potato rounds in a single layer and cook until golden, about 4 minutes. Flip and cook for 4 minutes more. Pour vegetable broth over the sweet potatoes and sprinkle with thyme, salt, and pepper. Close and lock lid. Select high pressure according to manufacturer's instructions; set timer for 2 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 lid.