To reproduce: 1. Create a new layout 2. Change Units to SI Millimeters 3. Save 4. Edit the same layout 5. It's back to PostScript Points This bug could be related http://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=5181
This is true for the patron card layouts. Couldn't reproduce the issue for the label editor.
*** This bug has been marked as a duplicate of bug 14366 ***