I believe there is possibly a bug in the export course code in birch (and release), or more likely in the split mongo module store export code. When I create a course in birch (in split) with some never published draftcontent and then import it into a draft mongo module store in the aspen release I end up with all of my content being published and then also made into drafts. This even includes content that was published and had no drafts. I have also seen where entire verticals just disappear during the import that were there in a split course.
I know this is kind of a weird release to release bug in the wrong direction, but it strikes me that exports should always be as compatible as possible with older releases as it makes QA and testing extremely difficult for us (as well as migrating content from edx.org to our open edX installation).
FYI this is also occurring as of the release on edx.org this week and was discovered with an import into aspen from an export of 7.28.1x.
Create course in split on current release with a mixture of draft and published units and import into an aspen release version of the stack. I've attached a simple version of a course that results in all content showing as draft in aspen, but 7.28.1x is probably the more interesting example where unpublished units (specifically Week 7, Lecture 14, Segment 3) completely doesn't import at all.