Comment 5 for bug 1573982

The apparent cause seems to be lvm2 (2.02.133-1ubuntu8). From the Changelog (https://launchpad.net/ubuntu/xenial/+source/lvm2/+changelog)

lvm2 (2.02.133-1ubuntu8) xenial; urgency=medium

  * Drop debian/85-lvm2.rules. This is redundant now, VGs are already
    auto-assembled via lvmetad and 69-lvm-metad.rules. This gets rid of using
    watershed, which causes deadlocks due to blocking udev rule processing.
    (LP: #1560710)
  * debian/rules: Put back initramfs-tools script to ensure that the root and
    resume devices are activated (lvmetad is not yet running in the initrd).
  * debian/rules: Put back activation systemd generator, to assemble LVs in
    case the admin disabled lvmetad.
  * Make debian/initramfs-tools/lvm2/scripts/init-premount/lvm2 executable and
    remove spurious chmod +x Ubuntu delta in debian/rules.

 -- Martin Pitt <email address hidden> Wed, 30 Mar 2016 10:56:49 +0200

The initramfs-tools script does not activate all of the logical volumes and its detection is lacking in certain edge cases like mine.