Without the guest agent shutdown will work too. You just shouldn't enable "QEMU guest agent" in the VMs option and then forget to install the guest agend. With "QEMU guest agent" option not enabled PVE will shutdown the VM via ACPI.Shutdown will only work, if you have installed and activated the Guest Agent. The Agent is normally installed in any Linux distribution (but you have to activate is in VM options), but you have to install it by yourself on a windows machine.