Rinse 1 cup (175 grams) medium-grain white rice under cold running water in a fine-mesh sieve until the water runs clear; drain well.
Add the drained rice to the Instant Pot. Pour in 2 cups (500 ml) whole milk and 1 ½ cups (375 ml) water. Add 1 teaspoon ground cinnamon, ¼ teaspoon ground nutmeg, 1 teaspoon pure vanilla extract, and ½ teaspoon salt. Stir once to combine.
Close the lid and set the steam-release valve to SEALING. Select Manual/High pressure and set the timer for 20 minutes.
When the cook cycle finishes, let the pressure naturally release for 10 minutes (do not move the valve). After 10 minutes, carefully move the steam-release valve to VENTING to release any remaining pressure. Wait until the float valve drops, then open the lid away from your face.
Open 1 (14 ounce) can (397 grams) sweetened condensed milk and pour it into the pot. Stir thoroughly until the condensed milk is fully incorporated and the pudding is creamy.