I had a question which asked for the German translation of "They are eating bread." I tried "Sie essen Brot" and was told I should have included the article for "Sie essen das Brot." The following question asked for the German translation of "You are eating bread." Based on the previous question, I tried "Du isst das Brot" and was told I was wrong and "Du isst Brot" was the correct answer.

Is there a rule for when the definite article should be used, or is this a bug?