It's not a bug, exactly, but rather a limitation. The questions for the grammar are not automatically generated from data in the system - it's a manual (by hand) process of having the sentences written, checked, translated, then questions made (in the case of multiple choice).
Last year, we added...I want to say roughly 20-30 new grammar elements into the dictionary, including this one. However, at the time, we (well, I, in reality) did not have time to also have a native speaker write the sentences and then generate all the question data.
It is a goal to have all grammar be studyable, so it'll get there eventually.
