Debian's Live Installer has changed the way Debian interacts with LUKS volumes. Manjaro similarly uses this technique. It configures the system with an encrypted boot volume and uses Grub to perform the initial unlocking. After Grub unlocks, it uses a LUKS key on disk (in the root of the drive and also included in the initramfs) to re-unlock it automatically after the Grub hand-off to the Linux kernel.
Obviously this throws a wrench in how Mortar interfaces with the system. I'm working on a solution. So far having the /boot location already encrypted does save us a few steps, and the current code already overwrites the Debian "hand-off key" slot, but looks like we need to fiddle with the crypttab file a little bit at least.