Some differences in rendering or printing would be acceptable - what is not acceptable, however, is unintended corruption of existing documents.

E.g. if I open a word document in libreoffice (to do e.g. review and commenting), save it without any changes to the layout, then I'd expect the original author to have the same document layout as before... and that is not so. The same applies to LibreOffice Calc - opening and saving the document produces changes.

Can't you just have a unit test that verifies that reading and immediately writing a document should keep it completely unchanged, except possibly for metadata?