Boot loader stuck after first stage upgrading 11.2 to 12.0-RC2

After successfully upgraded three hosts from 11.2-p4 to 12.0-RC2 (amd64,
zfs, bios), I tried my luck with one of our production hosts, and ended 
up
with a stuck loader after rebooting with a new kernel (after the first
stage of upgrade).

These were the steps, and all went smoothly and normally until a reboot:

   freebsd-update upgrade -r 12.0-RC2
   freebsd-update install
   shutdown -r now

While booting, the 'BTX loader' comes up, lists the BIOS drives,
then the spinner below the list comes up and begins turning,
stuttering, and after a couple of seconds it grinds to a standstill
and nothing happens afterwards.

At this point the ZFS and the bootstrap loader is supposed to
come up, but it doesn't.

This host has too zfs pools, the system pool consists of two SSDs
in a zfs mirror (also holding a freebsd-boot partition each), the
other pool is a raidz2 with six JBOD disks on an LSI controller.
The gptzfsboot in both freebsd-boot partitions is fresh from 11.2,
both zpool versions are up-to-date with 11.2. The 'zpool status -v'
is happy with both pools.

After rebooting from an USB drive and reverting the /boot directory
to a previous version, the machine comes up normally again
with the 11.2-RELEASE-p4.

I found a file init.core in the / directory, slightly predating the
last reboot with a salvaged system - although it was probably not
a cause of the problem, but a consequence of the rescue operation.

It is unfortunate that this is a production host, so I can't play
much with it. One or two more quick experiments I can probably
afford, but not much more. Should I just first wait for the
official 12.0 release? Should I try booting with a 12.0 on USB
and try to import pools? Suggestions welcome.



Now that the /boot has been manually restored to the 11.2 state,
A SECOND QUESTION is about freebsd-update, which still thinks we are
in the middle of an upgrade procedure. Trying now to just update
the 11.2-RELEASE-p4 to 11.2-RELEASE-p5, the fetch complains:

   # uname -a
   FreeBSD xxx 11.2-RELEASE-p4 FreeBSD 11.2-RELEASE-p4
   #
   # freebsd-version
   11.2-RELEASE-p4
   #
   # freebsd-update fetch
   src component not installed, skipped
   You have a partially completed upgrade pending
   Run '/usr/sbin/freebsd-update install' first.
   Run '/usr/sbin/freebsd-update fetch -F' to proceed anyway.

So what is the right way to get rid of all traces of the
unsuccessful upgrade, and let freebsd-update believe we are cleanly
at 11.2-p4 ?  Removing /var/db/freebsd-update did not help.

   Mark