"My grandmother eats cake without sugar."
Translation:Bunica mea mănâncă tort fără zahăr.
It is a fair point. However I noticed duo tends to be more accepting as otherwise you would have lots of people complaining, especially natives doing this course.
It is safe to accept bunica instead of bunica mea. I believe however that it wouldn't be right to accept bunica mea instead of bunica as it would narrow the interpretation.