No. It’s not a bug, it’s a feature. When you choose to export descendants, you only get descendants, and in most cases, incest aside, most spouses are not descendants. What I mean is, that when I export descendants of my father, the export will include me and my sister, because we are his descendants, but my mother is not, so she will not be included.
This may sound quite autistic, and I am an autist, so I can say that, but what you need is the export that says ‘family descendants’. That one exports spouses too, and I bet that it is what most users expect. Me too!