I've seen that happen, but every time there was some other obstruction in addition to the coffee: popsicle sticks, silverware, pencils, etc. Usually something too long to make it around the trap. Those things will eventually glob up with hair or worse and clog the drain anyway.
In my experience if you wash grounds down the drain with plenty of water there's never any problem.
Another common myth among moka machine coffe drinker is that ground coffee is somehow "good" for your drains. I believed it naively until my landlord came and unclogged a ground coffee-filled drain.
In my experience if you wash grounds down the drain with plenty of water there's never any problem.