The [tag:dash] tag is currently a synonym for the [tag:unity-dash] tag, so all questions that are tagged with [tag:dash] are automatically retagged with the [tag:unity-dash] tag instead. Ubuntu 17.10 and later also has an applications dashboard which is also called the Dash. Although the new Dash looks the same as the old Dash, there is a new tag for it called [tag:gnome-dash] instead of one tag for both. 

This is not the expected behavior. It is awkward and cumbersome. A better way to do it would be to convert [tag:dash] from a child synonym of [tag:unity-dash] into its parent tag and make both [tag:unity-dash] and [tag:gnome-dash] synonyms of [tag:dash].