This is a bug of the METS editor, but I can't open an issue of it.
If I use the DFG-Viewer without permanent stored and edited mets.xml file over the MCRMETSServlet the mods section will be generated cause in it two lines of data exists:
This work so long I does not use the METS editor. The editor stores the modified data als mets.xml file but it remove this both lines. So I don't get MODS information in the DFG-Viewer.
The correct DFG-Viewer is very impordent for DFG projects!!!