I'm having a similar issue with an RX480:
Asus X370-PRO
Ryzen 7 1700
NVIDIA GT1030 for unraid
Gigabyte RX480 8GB passthrough to macOS VM
NVIDIA GTK1070 passthrough to windows VM
Unraid 6.7.2
MacOS Mojave
"About this mac" lists the RX480 and I even see clover and a bit of the apple boot logo when I start the VM. The screen then goes to black and I can only access the VM using a viewer (NoMachine)