Drain canned clams, reserving the juice. Set aside.
Turn on the saute function on Instant Pot. Cook bacon until crisp. Add butter, onion, and thyme. Cook until onion is softened and translucent. Add garlic and cook for 1 more minute.
Add clam juice to a measuring cup. Add water until you reach 2 cups.
Add potatoes, clam juice, and salt and pepper to taste. Cook on manual high pressure for 4 minutes. Allow pressure to release naturally for 4 minutes, then do a quick release.
Turn the Sauté function back on. Add clams and half-and-half. Stir until combined. Stir in cornstarch, and stir until thickened.