Nice to see a bit of action in this thread after a few weeks. It seems both of use are using Ivy Bridge CPU's, I wonder if the issue is related to our CPU's or chipsets?
I notice in the Reddit thread you posted it mentions something about the E3-12xx CPU's:
IMPORTANT ADDITIONAL COMMANDS
You might need to add additional commands to this line, if the passthrough ends up failing. For example, if you're using a similar CPU as I am (Xeon E3-12xx series), which has horrible IOMMU grouping capabilities, and/or you are trying to passthrough a single GPU.
These additional commands essentially tell Proxmox not to utilize the GPUs present for itself, as well as helping to split each PCI device into its own IOMMU group. This is important because, if you try to use a GPU in say, IOMMU group 1, and group 1 also has your CPU grouped together for example, then your GPU passthrough will fail.
Is it possible that same GRUB config is required here? Sorry, I'm probably not much help when it comes to hardware troubleshooting on Linux.
GRUB_CMDLINE_LINUX_DEFAULT="quiet intel_iommu=on iommu=pt pcie_acs_override=downstream,multifunction nofb nomodeset video=vesafb:off,efifb:off"
I don't have the faintest idea of what any of this does or if it could help.