1) set up local staging site
2) add a document to "staging"
3) publish to remote but do not check document library - make sure that publish is from "last publish date"
4) publish again with documents selected - also be sure to use "last publish date"
5) observe that only the document added after the publish in step 3 is staged instead of all the documents since the document library was published.
This issue is only solved for local staging site. For remote staging see issue