I can't find any definitive answers to this question online. All the discussion I could find are speculation.
Factors that cause soy protein to coagulate are heat, acidity, and the presence of magnesium or calcium salts. Additionally the proteins are more likely to curdle if they are heated very quickly.
I will assume that you heated your coffee and milk identically with and without the salt. If you had changed the order of adding boiling water, for example, it could have prevented your curdling.
If you have hard water then there will be a good amount of calcium salts in your water. Coffee itself also has a good bit of magnesium. These salts will coagulate soy proteins more readily than acidity.
Sodium chloride does not cause soy proteins to coagulate. Sodium chloride ions will replace calcium ions- this is used in water softeners and soaking beans.
My suspicion (which to prove would require more experimenting than I have time for right now) is that the sodium ions are preventing the calcium and magnesium ions from coagulating your soy proteins.
Hmmm.....
Well, I've lived in India for 12 yrs & this is how I make 'masala chai' or 'spiced milk tea' at home & in my restaurant-
2 cups water
1 teaspoon black tea, loose leaf (I use Tokla tea from Nepal)
1 tablespoon grated fresh ginger
10 black peppercorns, coarsely ground
500 ml full fat 'whole' milk
Sugar to taste
In a stainless steel 3L pot combine tea, water, ginger & black pepper, bring to boil over high heat. Allow to boil for about 2 minutes.
Add milk to boiling tea, again allow to come to a boil. (Watch the pot closely in case the milk begins to foam, if it does foam- stir & decrease heat a little) Allow to simmer uncovered for 6 minutes.
Remove from heat, let stand for 2 minutes. Add sugar to taste if desired & stir. Use a tea strainer to strain into cups to serve. (or serve sugar on the side so people can add sugar to their own tastes)
Some notes-
I think you are trying to make 'tulsi' chai?
Tulsi = Indian holy basil
If so usually the fresh tulsi leaves (around 8-10) are added to the boiling tea mixture with around 8 green cardamoms, 2 cloves, 1/4 tsp fennel seeds, and a pinch of cumin seeds as I recall. Some Indians dry roast the whole spices to mellow the flavor a bit.
Use spices like fennel, cloves, star anise, cumin sparingly as their strong flavors can take over & give the chai a 'mouthwash' taste which isn't pleasant.
Best Answer
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