We believe that your issue has been closed by the upload of
Version 0.76 of grml-debootstrap from Michael Prokop <mika@grml.org>.
The explanation is attached below

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Hash: SHA256

Format: 1.8
Date: Thu, 22 Sep 2016 14:25:51 +0200
Source: grml-debootstrap
Binary: grml-debootstrap
Architecture: source all
Version: 0.76
Distribution: unstable
Urgency: medium
Maintainer: Grml Team <team@grml.org>
Changed-By: Michael Prokop <mika@grml.org>
Description:
 grml-debootstrap - wrapper around debootstrap for installing pure Debian
Changes:
 grml-debootstrap (0.76) unstable; urgency=medium
 .
   * [12f4a35] Hide errors during loading of efivars module
   * [5e6671f] packer: explicitly set Bash as default shell for Vagrant
     user
   * [8609e64] packer: apply predictable network interface workaround for
     Debian/stretch and newer
   * [ff11fa2] Disable ^metadata_csum' feature when creating ext fs for
     releases <= jessie [Closes: issue2178]
   * [b12a2ae] Ignore packer/packer_cache during dpkg-source generation.
     Thanks to Guillem Jover for the hint
   * [987bf63] Switch Homepage + Vcs-Browser fields to https
   * [6a665b1] Bump Standards-Version to 3.9.8
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Files:
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Tested with grml64-small_testing/2016-04-03_06-01-41.

grml-debootstrap with rootfs set to create as ext4.

After rebooting, e2fsck complains:

Apr 04 22:58:21 zyl-vn01 systemd-fsck[199]: /dev/mapper/datastore-root has unsupported feature(s): 
metadata_csum
Apr 04 22:58:21 zyl-vn01 systemd-fsck[199]: e2fsck: Get a newer version of e2fsck!