Can't mount crypted RAID1 disks after stable install from buster netinstall

[Rory Campbell-Lange <rory@campbell-lange.net>]

I've installed a system with / on two disks in software RAID1 with
dm_crypt on top from a buster netinstall disk 20190429-03:57.

I'm using a buster netinstall disk because the machine's ixgbe network
adapters aren't supported under stable.

After a stable install and rebooting, it isn't possible to unencrypt
the RAIDed volume. I've ensured the password is correct by copying and
pasting on a serial console connection and also trying many times by
keyboard and ipmi virtual keyboard. I've also tried the reinstall over
5 times with slightly different options including ensuring all available
kernel modules are installed.

Mounting works fine if I install buster rather than stretch with exactly
the same configuration.

Two questions:

* could there be a problem in the stable component of the installer
  for grub or other mising support for dm_crypt?

* since I'm aiming to run stable on this machine but I need recent ixgbe
  network modules anyway, can I install the kernel-image-4.19.0-0.bpo
  backport package at the end of the installation process from a console?
  I'm hoping this will also solve the mounting problem I'm experiencing.

Thanks very much
Rory