I just downloaded an older BIOS on their website and I was able to install it as you would a newer version. Apparently my motherboard didn't bother downgrading.
I had the exact same problem using a 1600X, a 1660Ti and an Aorus X470 Ultra Gaming until I downgraded the BIOS To version F6.
I think the update to a newer AGESA version to support the upcoming CPUs on AMD platform is breaking the passthrough capabilities. I've seen posts of people complaining about this exact issue with other motherboard and a downgrade seems to fix it in most cases.