This problem still persists, on DI Buster Alpha 2 and Alpha 3. Working around this even as an expert is rather cumbersome; one has to play with the various menu options, partially configuring disks/partitions etc., to load the necessary cryptsetup components (and dependencies, like awk for decrypt_derived script), and it cannot be completed via the installer.
As the submitter notes, having /boot on an encrypted (LUKS) partition _is_ supported by GRUB and the other softwares. Simply making "continue" not do the same as "go back," as was originally suggested, would be a big improvement. Without this, one has to setup with an unencrypted /boot, and then use rescue mode or other knowledge to encrypt /boot and finalize the setup. I found no way to proceed to install with /boot being encrypted. -- Nate