Drain clam juice from 2 cans into a 2 cup liquid measure, then fill with water to the 2 cup mark; set aside. Drain and discard clam juice from the remaining can.
Turn on a multi functional pressure cooker (such as Instant Pot) and select the Sauté function. Add bacon and stir occasionally until fat has rendered and bacon is cooked, but not crispy, about 5 minutes. Add onion and butter. Cook, stirring and scraping the bottom of the pot to release the brown bits, until onion has softened and turned translucent, about 5 minutes. Hit Cancel.
Add potatoes and clam juice water mixture; stir. Close and lock the lid and seal the pressure valve. Select high pressure according to manufacturer's instructions; set the timer for 4 minutes. Allow 10 to 15 minutes for pressure to build.
Allow to rest for 3 minutes, then release pressure carefully using the quick release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid.
Carefully mash some or all of the potatoes as desired with a potato masher. Select Soup function. Stir in flour, thyme, garlic powder, salt, and pepper. Add evaporated milk and drained clams.
Heat chowder without boiling, about 5 minutes. Stir in potato starch to thicken.
Easy Instant Pot Clam Chowder · Dinner Match Lab | Dinner Match Lab