Comment 1 for bug 47767

Looks like Joey fixed this in d-i upstream r35026:

Index: debconf-set-selections
===================================================================
--- debconf-set-selections (revision 35025)
+++ debconf-set-selections (revision 35026)
@@ -24,7 +24,7 @@
                multiline="$multiline${line%\\\\}"
                continue
        elif [ -n "$multiline" ]; then
- line="$multiline"
+ line="$multiline$line"
                multiline=""
        fi

Would you mind testing this patch? If you boot with the BOOT_DEBUG=3 kernel option, then you can use nano to edit /bin/debconf-set-selections on the fly. If that works, we may be able to get it into dapper-updates.