Hello.

When system boot form iSCSI multipath device, rootfs mounted as ordinary scsi disk.

:~# cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 9 (stretch)"
NAME="Debian GNU/Linux"
VERSION_ID="9"
VERSION="9 (stretch)"
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"

:~# cat /etc/debian_version
9.6

:~# dpkg -l | grep multipath-tools-boot
ii  multipath-tools-boot           0.6.4-5+deb9u1                 all          Support booting from multipath devices
  .


Suggested solution
--- /usr/share/initramfs-tools/scripts/local-top/multipath.orig 2018-07-23 12:16:34.000000000 +0600
+++ /usr/share/initramfs-tools/scripts/local-top/multipath      2018-12-13 13:42:06.037168125 +0600
@@ -42,6 +42,9 @@
  done
  verbose && log_end_msg

+if [ -x /bin/udevadm ]; then
+    /bin/udevadm settle --timeout=10
+fi
  verbose && log_begin_msg "Discovering multipaths"
  /sbin/multipath -v $VERBOSITY
  verbose && log_end_msg


Regards.