Hey, 6 years past, on bullseye this time, and the same bug remain.
No way to preseed the keyboard selection, only way I found is to skip it using "keymap=fr". Doing so will also skip any keyboard selection. So I needed to set it manually after the installation was done using localectl. It didn't work. it didn't work because /etc/X11/xorg.conf.d/00-keyboard.conf was set and kept overwriting any change from localectl.
When I asked around me, nobody heard of it. Removing that file makes localectl efficient again.
Story (in french) about me being very slow at finding out here
https://debian-facile.org/viewtopic.php?pid=410530