However I just did a miminal Kubuntu install in a VM, and I can confirm the problem. The thunderbird package is marked as installed, but neither /usr/bin/thunderbird nor /usr/share/applications/thunderbird.desktop are present on the system. It's as if they had been manually deleted without interaction with apt.
Nothing relevant in this syslog, unfortunately.
However I just did a miminal Kubuntu install in a VM, and I can confirm the problem. The thunderbird package is marked as installed, but neither /usr/bin/ thunderbird nor /usr/share/ applications/ thunderbird. desktop are present on the system. It's as if they had been manually deleted without interaction with apt.