Debian Buster upgrade boot failure (LVM + BTRFS)

Upgraded one of my servers to Buster and was faced with an unbootable box, unable to find the rootfs (and 'open_ctree failed'). On the server in question I am running a filesystem of BTRFS RAID 1 across two independent LVM disks.

Faffed around for ages trying to diagnose, in the end had success with the nuclear option:

Create /etc/initramfs-tools/scripts/local-top/forcelvm:

#!/bin/sh
PREREQ=""
prereqs()
{
   echo "$PREREQ"
}
case $1 in
prereqs)
   prereqs
   exit 0
   ;;
esac
. /scripts/functions

lvm vgchange -ay

btrfs device scan

Chmod it +x

Then update-initramfs -u -k all