FreeBSD reboots but does not power off

[Matthew Woehlke]

Per https://www.freebsd.org/doc/handbook/acpi-overview.html, I am
posting here before filing a bug report.

Using 12.0-RELEASE r341666 GENERIC, I am unable to power off my system
(i.e. `poweroff` or `shutdown -p now`); it is necessary to hold the
power button until the BIOS cuts power. However, rebooting works.

CPU: AMD Ryzen Threadripper 1920X
Motherboard: Gigabyte X399 Aorus Pro
BIOS: ID=8A07BG04, Version=F1, Date=2018-10-01
Kernel: 12.0-RELEASE r341666 GENERIC

Linux (Fedora 29) is able to power off the system, so presumably it is
not a hardware problem as such.

Output of `dmesg`, `sysctl hw.acpi` and `acpidump -dt | xz` is attached.
Please let me know what other information I can provide or what
experiments I can try.

-- 
Matthew