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.

My instant pot isn't consistent in temperature for saute or high heat. I heat up the milk in a separate pot and use an instant read thermometer to ensure it hits the correct temperature.

When you mix the culture into a small part of the milk, make sure to get that to a fairly smooth consistency before adding it to the rest. That also seems to help achieve a smoother result.

I use my Instant Pot in pretty much exactly this way, but have never had lumpy yogurt, so maybe something else?

That's good to know, thank ou. Maybe it's my starter culture. I had been using yogurt from Dannon or a Skyrr.

Try (if you can find it!) Stoneyfield plain yogurt, not their Greek-style. It tastes great, and I have a 100% success rate with that as a starter. Good luck!