Comment 13 for bug 1619844

This bug disappeared for me when I did

apt-get install linux-generic-hwe-16.04

to upgrade to kernel version 4.10.0 instead of 4.4.0. (I also had proposed-updates in /etc/apt/sources.list, but it was the linux-generic-hwe-16.04 package that really fixed it.)

I suspected it was a kernel bug after I installed upstart-sysv instead of systemd-sysv and the bug still persisted, and I also tried editing the upstart scripts to do "echo o > /proc/sysrq-trigger" instead of normal poweroff, whereupon the kernel reported the SysRq trigger event but didn't act on it. Then I tried the newer kernel and everything worked fine (with my other changes reverted) regardless of whether I was using upstart-sysv or systemd-sysv.