Since ephy 3.6, if you right-click on the dash app icon and select "open new window", it opens a new tab in a currently opened epiphany window. This is particularly problematic if you try to open a new window of epiphany in another workspace, for example by dragging and dropping the dahs icon to another workspace, as it simply does not work anymore: it always creates a new tab in an opened epiphany window instead of creating a new window in another workspace.

PS: it works well, however, using CTRL-N from epiphany, or by cliking on "new window" from the app menu in the top bar.