Comment 70 for bug 1267059

Happens on Trusty and Xenial too. There are a few bug reports about it marking kernels as manually installed. They may be related. Here's a test case anyway:

Normal run:

# apt-get install lolcat
# apt-mark showmanual lolcat
lolcat
# apt-mark auto lolcat
lolcat set to automatically installed.
# unattended-upgrade
# dpkg -l lolcat
dpkg-query: no packages found matching lolcat

With --dry-run:

# apt-get install lolcat
# apt-mark showmanual lolcat
lolcat
# apt-mark auto lolcat
lolcat set to automatically installed.
# unattended-upgrade --dry-run
# dpkg -l lolcat
ii lolcat ...
# apt-mark showmanual lolcat
lolcat