Don't worked shutdown/reboot on Cinnamon.
You'll have to find the configuration file that tells the buttons what commands to execute, and use the following for shutdown and reboot, respectively:
/usr/bin/loginctl poweroff
/usr/bin/loginctl reboot
How to make pull-request to new git-server ? It fix same errors in Cinnamon.
You could maybe send a 'patch' to the mailing list.