Sorry to revive this topic... I was just wondering: which force flag are you using?
Per the docs I couldn't find it for xe host-shutdown command.
FYI (in case it could help), my idea is to implement UPS shutdown order via SSH by using force command in /root/.ssh/authorized_keys.
command="the_force_shutdown_command_here",no-agent-forwarding,no-port-forwarding,no-pty,no-user-rc,no-X11-forwarding a_dedicated_pub_key_here
Then if you ssh in, it will force the command command= (other command will be ignored and you can't open interactive shell).
ssh -i /path/to/dedicated_private_key user@xcp-ng-host whatever_command
(whatever_command is just there to prevent error message)