Why does alcohol curdle milk?
I'd say that when you add the milk-coffee mix to alcohol the concentrated alcohol must be able to denature the milk proteins initially (similar to acid in cheese making but a different mechanism - hydrophobicity), alcohol added to coffee/milk must not ever be concentrated enough to do this.
Interesting observation, reminds me of one I've seen - adding water to sauteeing onion or garlic makes them brown/burn more quickly (I've thought that this might have to do with the steam denaturing proteins more efficiently).
Did you learn in high school chemistry to add the acid to the water, not the water to the acid?
If you add the acid to the water, it doesn't happen in 0-time. The first drops of acid are happy because there is water everywhere! However, if you add water to acid, the acid molecules fight each other for the water...bad. (Forgive me for personifying water and acid.)
I suspect the same type of thing is happening with your milk.