The § character is appearing in IPTC metadata because the IPTC metadata was not written as Unicode.
To resolve this, first navigate to Photo Mechanic's Preferences-->IPTC/XMP, then select the "Write IPTC as Unicode" checkbox.
Open the folder of affected images as a contact sheet, then select all of the images.
With all of the images selected, from the top menu, navigate to Tools-->Update IPTC/XMP.
Select the "Update" button in the Update IPTC/XMP window that appears.
This will update the IPTC metadata to be written as Unicode.
Note: XMP metadata is always written as UTF-8/Unicode.