Why does soy milk curdle in coffee but not in tea?

I'm really confused by the question, being lactose intolerant I'm a heavy soya milk user, and being an english student with unreasonable project deadlines I find coffee essensial for those 3-days-solid-writing-no-sleep times. But, my coffee has never curdled.
I don't heat my "milk", I don't let my coffee cool and its a rare treat if the milk is under 5 days old. However when I add it to my cup of earl tea I get white floaty bits.
Maybe I'm just being awkward.

The acidity of the coffee coagulates the proteins in the soya milk.

Try shaking the milk before pouring and putting that in the cup first, pouring the coffee in to the milk as opposed to adding the milk in to the coffee. That's worked for me so far.

I am still in the process of finding out why this happens (why I'm on this page) but I have a far simpler solution than heating the soya milk to be hotter than the coffee.

Just put the soya milk in the cup first and pour the coffee on top. I only learned this recently, but so far (3 coffees) every cup has been swell.

I have tried a few coffee brands and found that Kenco does not make the milk curdle.

If you heat the soymilk in the microwave before you put the coffee in this won't happen even with older soymilk. I have no idea why it works, but it does. It could just be a different PH of the tea, maybe a nut milk would work better? i think it's a combo of hot/cold and pH. i don't think it makes it bad tho? just mix it up and as the coffee cools it usually goes into solution smoothly (i've found).

soya milk will always curdle in instant coffee. For ground coffee the milk has to be hotter than the coffee, so if you only warm your milk it will still curdle. I heat the milk to boiling then add to slightly cooled coffee. Works every time.

Thank you for the answers. I only discovered this yesterday. I have been having soy milk for 15 years but I drink black coffee so I have never noticed it. My daughter in law is a soy convert and last night I noticed the milk curdled in her coffee, I made another one and it did the same, tried a new soy milk and it still happened. She advised me that this always happened when she made it. Now we know, thanks for the tips above.