There are two reasons why the soy milk curdles:
Don't worry, though – there's a solution.
And if you're making a big pot of coffee at home, dilution is your friend.
Jeremy Challender says: "Curdling's less likely to happen in a more diluted beverage, so if you did a big 60g/litre pour-over, the pH will be a bit higher than if it was a super strong stove-top espresso. So slightly more dilution should help with your coffee at home."