CentOS - Feb 2020 - Nested Virtualization with CentOS 7 host and CentOS 8 guest

I'm not sure whether this is a specific CentOS question, but I'm hoping
someone here can give me some pointers.

I have an OpenStack compute node running CentOS 7.4.1708 and kernel
3.10.0-693.17.1.el7.x86_64 which is configured for nested virtualization; this
has been set up for a few years and works fine when running CentOS 7 guests
which themselves are using virt-create / virt-customize etc.
Nova is set up on the compute node so that guests have cpu mode
host-passthrough.

I have been trying to run a CentOS 8 guest on the compute node so that I can
build CentOS 8 images for OpenStack (there is an issue using a CentOS 7 server
to build a CentOS 8 image related to xfs options which are only supported
read-only by CentOS 7).

However, every time I try to build a CentOS 8 image using a CentOS 8 guest, the
CentOS 8 guest crashes with a kernel panic, not immediately but usually near the
end of the image build process. (I have also seen similar behaviour using a
Fedora 31 guest to build a CentOS 8 image, but the crashes are less frequent,
and some image builds have succeeded).

The CentOS 8 guest is running CentOS 8.1.1911 (Core) and kernel
4.18.0-147.3.1.el8_1.x86_64.

Any ideas?