Comment 12 for bug 1554099

It seems unattended-upgrades sets default to *install* in 20auto-upgrades, (1), but in 10periodic the default for download (APT::Periodic::Download-Upgradeable-Packages) is set to just Display immediately (0)

This creates an impossible situation, Unattended-Upgrade can't install when Download is off

Once the 10 file is edited to make the 20 file viable any changes in Software & Updates regarding these options is mirrored between the 2 files so they are 'on the same page' so to speak from that point on.

The whole deal can be re-created at any point by removing unattended-upgrades package, then re-installing software-properties-gtk & unattended-upgrades