TitlesMultiselectWidget::$showMissing is documented as "Show missing pages" so you'd expect it to enable/disable displaying non-existent pages in the page list of the widget. What it actually does is enable/disable showing the exact title you have typed in in the typeahead dropdown, so basically it controls whether it is possible to add non-existing pages. (The configuration setting is eventually passed to TitleWidget, where the naming makes sense, but here it doesn't.)

It would be nice to rename it to something more intuitive, like canAddMissing.