Thank you for posting this. I was having the same issue with this motherboard ( ASRock B550 Taichi Razer Edition ATX AM4 Motherboard ). Deleting vfio-pci.cfg corrected the issue.
I tracked down the topic using the google search term: unraid iommu eth0 not detected
Hopefully the added keywords will pull this up for someone in the future.
Has anyone found a solution? I am running into the same issue after adding a rx 570 to a bigsur vm following spaceinvaders tutorial.
Motherboard:ASRock B550 Taichi Razer Edition,
Processor:AMD Ryzen 7 3700X 8-Core @ 3600 MHz
I have followed all the passthrough guidance (BIOS and system devices) that I could come across with dozens of reboots and two installs of the VM.