Just to let you guys know (as this was the most helpful thread I've found related to this issues), this is not an issue only in UnRaid. I'm currently using KVM on ubuntu 18.04 and encountering the same issues. PCI passthrough doesn't throw any obvious errors using Virt-Manager but nothing happens when trying to boot a machine that already worked in previous BIOS versions. Also, when I force off the VM, it starts throwing the header 127 error.
Specs:
Asus Prime-x370-pro
Ryzen 1700
BIOS Version 5220 - AGESA 1.0.0.3ABBA
Passthrough: GTX 1080 + Via Technologies PCIE USB board
Me using an Nvidia Card and having the same issues kinda negates the AMD reset bug
Things I tried:
Updated Kernel to 5.4.5 with ACS patch
Reassigned the board (as the IDs changed when upgraded BIOS)
BIOS Version 5204 - AGESA 1.0.0.3AB
All options lead to the same issues.
AMD is doing some very nice work on supporting the AM4 socket for a long period and adding support for the new chips on reasonably old chipsets but I think we all can agree it's not nice on them to break things that worked on older versions to be able to do that.
Also, on the Board manufacturers side (ASUS mainly in my case), there is no reason we can't have a reasonably straightforward and official way to downgrade our BIOS.