Comment 46 for bug 1464917

Xenial is also affected: reboot & shutdown, the lock usually ends by itself 10_20 seconds later, but sometimes it does not ends at all.

Looks like its due to "lockfile-progs" which is described as:

*******
These programs use liblockfile to perform the file locking and
unlocking, so they are guaranteed compatible with Debian's
file locking policies.
*******

Is there a Debian/Ubuntu file locking policies difference explaining that lock ?