It's better to use something to mix the formula rather than shaking it. You'll find it will mix much better.
I haven’t had the issue but have seen a few people mention it. I think if it’s added to a little bit of boiling how water and mixed/ swirled in the bottle then it’s fine and then all cool water after to bring the temperature down to what’s needed to feed baby. That’s how I do it and no clumps.
I had to is issue with the organic cows milk one. I’m using the goat milk formula now and I don’t have that issue at all. I did Use a whisk to try and get the clumps out and it seemed to help