The autostart file from hplip-gui
is located in /etc/xdg/autostart/hplip-systray.desktop
.
To fix the menu display we need to launch hplip with help of DBus (as in DropBox case) by creating a custom copy of this file for the user:
killall hp-systray
rm ~/.hplip/hp-systray.lock
mkdir -p ~/.config/autostart/
cp /etc/xdg/autostart/hplip-systray.desktop ~/.config/autostart/
sed -i "s/hp-systray -x/dbus-launch hp-systray -x/" ~/.config/autostart/hplip-systray.desktop
and then reboot and enjoy:

Notes:
This method is useful for 18.04 LTS, 18.10, 19.04 and 19.10. It is a temporary fix for bug 1810745.