The second file, the version that was published, is not actually a JPEG, but a webp file with a JPEG extension. If this Blox software is creating the webp file from the JPEG, that I would guess it is dropping the metadata at the time that it is creating the file. I can't really imagine why it would different for different cameras unless you were changing settings in Photo Mechanic's IPTC/XMP preferences. If you want to send us one of the files from the D5 we could see if there is something different about that and maybe make a suggestion, but I would imagine it would be more efficient to ask the makers of that software why it is losing the metadata when making a webp from the jpeg. If the editor wants to confirm that the metadata is present in the file that you are sending them, but they don't have Photo Mechanic, they'll hopefully have some other software on their computer that can view the metadata such as macOS Preview or Photoshop