By default Ubuntu 17.10 alt-tabbing works across all workspaces, to change this
run the following command in a terminal (CTRL-ALT-T)1
dconf write /org/gnome/shell/app-switcher/current-workspace-only 'true'
To change it back to default behavior run
1 | dconf write /org/gnome/shell/app-switcher/current-workspace-only 'false' |