I have managed to boot successfully by making a bootable FreeDOS drive with Rufus and copying the files for UnRAID 6.9.2 onto the drive and running makebootable. So it appears to be a problem of UnRAID 6.10.2
I have the exact same issue with a Ryzen 3700X and Asus B550I Strix. Latest bios and Unraid 6.10.2. Boots Windows and TrueNAS perfectly fine, it’s just Unraid which doesn’t boot. I’ll give your solution a try this afternoon.