I'm going to take a shot at this. Here's how interaction between egg and coffee was explained in an article on putting egg in coffee:
"...the proteins bind irreversibly to astringent and bitter tasting polyphenols in coffee to form insoluble complexes that will precipitate."
Even egg white is 10% protein. Whatever those insoluble complexes might be, it might be that having that happen causes some separation and might certainly defeat the egg white's function.
But I also find recipes with espresso powder, plain instant coffee, and ground espresso. Some are rather flat with cracks. Many are plump and smooth. I'm working from a limited sample here, just those with both recipe and photo. But I believe I see that the flat cracked results come from recipes with large volumes of coffee powder, like 6 tsp or 1 tbs for three or four egg whites. The plump smooth ones use one or two tsp. So maybe it's just too much coffee powder for the amount of liquid and they're just dry.
