How to prevent homemade soy milk separating in hot tea?
The problem you encounter is that soy milk curdles when coming in contact with high acidity and high temperatures. Sadly, it's a chemical reaction that will almost always happen when soy milk comes in contact with high temperatures and acidity. If you want your soy milk to incorporate nicely, try finding a way to lower the acidity, or try to lower the temperature of the tea before you pour the soy milk into it.
You can also try pouring the soy milk first or add a sprinkle of salt.

Please note that no matter what, high temperatures usually result in coagulation when in contact with acidity.

Source:
https://coffee.stackexchange.com/questions/425/how-to-avoid-soya-milk-curdling-in-coffee