brent Thanks for looking into it. I have followed the article but have had no luck so far.
I found this command work from terminal
dbus-send --system --print-reply --dest="org.freedesktop.login1" /org/freedesktop/login1 org.freedesktop.login1.Manager.Suspend boolean:true
But setting up a system shortcut using this command has the same issue. I think some magic that Power Dialog works differently compared to the setup from system shortcut. At least I found this shortkey <Super>+<Shift>+P to bring up Power Dialog. I will live with it for now.