Thank you for raising this again and for your research Thomas. I now see that my understanding of the specification was wrong. I'll revert changes to zipfile.

There is one question. If the zip file was concatenated to other file (and looks incorrect from the view of other tools), and we opened it in append mode and added new entries, should the central directory count offsets from the start of the file or from the start of embedded archive? In the first case offsets of first entries (which was in the original concatenated archive) in the central directory will be different from offsets in local headers. In the second case the zip file will look incorrect from the view of other tools (but it already looked incorrect before appending).