MERGED_USR=yes breaks debootstrap of updated xenial distrib

The latest change of MERGED_USR=yes in bionic is causing failure to debootstrap an updated xenial chroot on bionic.

We have a mirror of ubuntu apt repositories where xenial, xenial-updates and xenial-security are merged into one distribution called xenial-updated-bootstrap. We do this to be able to debootstrap a fully up to date chroot.

The debootstrap command looks like this:
debootstrap --arch=amd64 --components=main --include=SOME_PACKAGES xenial-updated-bootstrap target/ http://mirror /usr/share/debootstrap/scripts/xenial

This results in the debootstrap failing with these errors:
W: Failure while installing base packages. This will be re-attempted up to five times.
W: See /data/foober/debootstrap/debootstrap.log for details (possibly the package /var/cache/apt/archives/kbd_1.15.5-1ubuntu5_amd64.deb is at fault)
I: Unpacking kbd...
W: Failure while installing base packages. This will be re-attempted up to five times.
W: See /data/foober/debootstrap/debootstrap.log for details (possibly the package /var/cache/apt/archives/kbd_1.15.5-1ubuntu5_amd64.deb is at fault)
W: Failure while installing base packages. This will be re-attempted up to five times.
W: See /data/foober/debootstrap/debootstrap.log for details (possibly the package /var/cache/apt/archives/kbd_1.15.5-1ubuntu5_amd64.deb is at fault)
W: Failure while installing base packages. This will be re-attempted up to five times.
W: See /data/foober/debootstrap/debootstrap.log for details (possibly the package /var/cache/apt/archives/kbd_1.15.5-1ubuntu5_amd64.deb is at fault)
W: Failure while installing base packages. This will be re-attempted up to five times.
W: See /data/foober/debootstrap/debootstrap.log for details (possibly the package /var/cache/apt/archives/kbd_1.15.5-1ubuntu5_amd64.deb is at fault)

In the debootstrap log I find these:
Unpacking kbd (1.15.5-1ubuntu5) ...
dpkg: error processing archive /var/cache/apt/archives/kbd_1.15.5-1ubuntu5_amd64.deb (--unpack):
 unable to open '/bin/dumpkeys.dpkg-new': No such file or directory
Unpacking libpng12-0:amd64 (1.2.54-1ubuntu1.1) ...
dpkg: error processing archive /var/cache/apt/archives/libpng12-0_1.2.54-1ubuntu1.1_amd64.deb (--unpack):
 unable to install new version of '/lib/x86_64-linux-gnu/libpng12.so.0': No such file or directory

Even if I add --no-merged-usr option (which is supposed to be the default according to the man page) the debootstrap fails.

The version previously available in bionic works fine to debootstrap the updated xenial distribution.