I've been using unattended upgrades without any problems until now. However, for the second time, I'm encountering an issue where I end up with a summary stating that some packages were not installed or updated. Among other things, it tells me:

Package imagemagick is held back because a related package is held back or due to local apt_preferences(5).

However, when I run sudo apt full-upgrade, everything is up to date and the packages are installed. sudo apt-mark showhold also doesn’t list any packages. So why am I getting the message that packages were held back?