Good job figuring it out. If updating UEFI(that's a UEFI board, not a BIOS one, though it's not much relevant here) and re-enabling the PCI-E resume still exhibits the issue, you should report a bug, presumably to the kernel.
UEFI upgrade was the first thing I've done. It did not help, although it removed a lot of ACPI warnings about conflicting regions from dmesg.
I am sure it is a kernel, broken "poweroff -f" do not interact with systemd AFAIK.
Regards,
Łukasz
