/usr/lib/base-installer/library.sh is missing a line to let
preseed-directive
"AllowUnauthenticated" work. See patch.

Also see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1101148

PR 1101148 is filed under package debian-installer, but the file to be
patched
is in package base-installer. Perhaps someone should merge the bug reports.
Or put references to each other.

Markus