The Instant Pot is routinely used for making Greek Yogurt and collecting the resulting whey. The best luck has been with 1/2 gallons of milk and the medium temperature setting. Spotty results when doing a whole gallon at a time. Maybe the heat doesn't get through the milk evenly or the starter culture doesn't stay mixed in well. Getting the starting culture mixed in well seems to be important for the outcome. Insufficient mixing leads to clumps of setup yogurt and takes longer for the rest to setup.