Indeed, the log shows D-Bus was upgraded during the process, so maybe it's the install script that messes with GIDs...
upgrade libdbus-1-3 1.2.16-2ubuntu4.1 1.2.16-2ubuntu4.2
upgrade dbus 1.2.16-2ubuntu4.1 1.2.16-2ubuntu4.2
upgrade dbus-x11 1.2.16-2ubuntu4.1 1.2.16-2ubuntu4.2
Indeed, the log shows D-Bus was upgraded during the process, so maybe it's the install script that messes with GIDs...
upgrade libdbus-1-3 1.2.16-2ubuntu4.1 1.2.16-2ubuntu4.2
upgrade dbus 1.2.16-2ubuntu4.1 1.2.16-2ubuntu4.2
upgrade dbus-x11 1.2.16-2ubuntu4.1 1.2.16-2ubuntu4.2