Critical battery level triggers Log Out dialog, not battery warning dialog

In xfce Power Manager > System > Critical Power
I have "On critical battery power" set to "Ask"

As soon as the critical level is reached a dialog pops up, though it's the regular Log Out dialog,
not the power manager's battery warning dialog:

xfce4-power-manager* 1.6.6-1 xfce4-session 4.14.2-0ubuntu1 xfce4-settings 4.14.3-0ubuntu1