Cube is available in both MDX and tabular formats
So far this seems your biggest problem: thinking there is only one cube with two minor variations. In fact, multidimensional and tabular engines are internally vastly different, so what you have are two distinct BI solutions that you tried hard to make as similar from end-user perspective as possible.
The line with blank dimension member name most probably corresponds to the rows of the table you source your measure from that have no related rows in the table with the column used for the dimension.