Nutanix support has posted a KB article listing four (4) potential scenarios where the Windows configuration can result in the Operating System ignoring the Guest Reboot and Guest Shutdown power commands.

1. An active screensaver and/or display timeout is configured
2. Windows is not configured to Shut Down when the Power button is pressed
3. The Power Service is Disabled
4. Windows may require setting the Power button action to a forced shutdown 

Additional information is available via the Nutanix support article:
https://portal.nutanix.com/page/documents/kbs/details?targetId=kA00e000000LJZSCA4

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Active screensavers and/or specific power configuration options can cause Windows to ignore the ACPI request to perform the specified actions

Administrators using Prism should review the Nutanix support article and the associated Windows configuration to prevent power actions from being ignored by the OS.

Power actions may be ignored by Windows VMs running on AHV due to screensaver being active and/or the Windows power configuration prevents processing the ACPI request.

https://portal.nutanix.com/page/documents/kbs/details?targetId=kA00e000000LJZSCA4