Comment 38 for bug 1499556

I recently tested this with the latest ubuntu 16.04 beta 2 image and I have the same behaviour.

I've also tested with several variants of the acpi kernel parameter including acpi=off, acpi=strict, and acpi=oldboot none changed the behaviour except for acpi=off. When this parameter was set, I was able to press the power button to shutdown but the system shuts down _immediately_ and I'm fairly certain the OS hasn't even flushed to disk yet, so that's not a good workaround.