This occurs because Synaptic is looking for the <packagename>.list file in /var/lib/dpkg/info, but for packages that are multi-arch capable, the filename in /var/lib/dpkg/info contains a ":amd64", even if that is the native architecture of the machine. Proposed patch attached which makes an additional attempt to open a file with the architecture name embedded.
This occurs because Synaptic is looking for the <packagename>.list file in /var/lib/dpkg/info, but for packages that are multi-arch capable, the filename in /var/lib/dpkg/info contains a ":amd64", even if that is the native architecture of the machine. Proposed patch attached which makes an additional attempt to open a file with the architecture name embedded.