One of the first times I tried using an Instant Pot, I was also using a starter (powdered, commercial) I'd not used before. It turned to cheese curds. I came to the conclusion that the Instant Pot got too hot for that particular culture. So, yes - it could be a combo of both temp and culture causing your lumpiness. I've noticed too with a different culture (yesterday's yogurt) that even after mixing thoroughly, it seems like it clumps back together and I've not been able to discern why.