The "seen" flag of the unmount_active question is set to false which prevents preseeding the response. This breaks unattented installs. See LP: #1347726
Index: partman-base-172ubuntu1.1/init.d/parted
===================================================================
--- partman-base-172ubuntu1.1.orig/init.d/parted	2015-07-17 12:00:37.000000000 -0400
+++ partman-base-172ubuntu1.1/init.d/parted	2015-10-22 08:29:56.613255350 -0400
@@ -236,7 +236,6 @@
 		done
 		if [ "$disks_unmount" ]; then
 			db_subst partman/unmount_active DISKS "$(echo "$disks_unmount" | sed 's/ /, /g')"
-			db_fset partman/unmount_active seen false
 			db_input critical partman/unmount_active || true
 			db_go || exit 10
 			db_get partman/unmount_active || RET=