Two different things can cause it: the acid content of the coffee and the temperature of the coffee.
If you use a drip coffee maker and are relatively careful measuring your grounds, the results will be consistent enough you see a change in behavior rarely, probably when you do something like change when you pour the coffee.
I use a French Press and I’m kind of lazy, so the amount of grounds I use and how long they steep changes a bit. I see my milk curdle a lot more often than 1/100.
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