Thanks to you for testing and reporting.
About what your feature request, if you export the annotated PDF to .. PDF will it do what you're looking for ?
As far as I know Xournal can save the annotated PDF as a PDF or as a journal + PDF.
The former means that the exported PDF is a regular PDF, while the latter means that the annotations are saved in a Xournal internal format applied to the original PDF when you open the journal (that is, it should be possible to modify them later).