Can't capture vmcore?

I'm getting kernel panics in a VM that functions as a hypervisor, the
moment I spin up the nested guest (on AMD ThreadRipper / Fedora 27). That
is annoying, of course, so I try to be a good citizen and file a bug.

For some reason though, I cannot get the core dumped. I get a core fine
with sysrq, but not with this actual panic. I've followed [1] to set up
kdump and crash, but everytime I trigger the crash and see my VM reboot, I
see an empty /var/crash afterwards.

As was able to get the vmcore written to /var/crash on in a RHEL7 guest,
I'm starting to suspect a bug, but I'm unsure.

Any pointers on how to debug this?

[1] <a href="" title=""></a>