Dear Tim,
I have the same HP Elitebook and experienced the same issue when upgrading jessie a few months ago. At the time I solved the issue by booting into an older version of the kernel from GRUB. But then a few weeks ago that workaround stopped working. I thought that completely reinstalling my system (now stretch) would help, but alas it did not.
Now I've discovered your bug report, but I'm still not sure what action I should take to get my laptop to power down. Do you have any advice for me?