Bug 791352 - Screenshot delay is broken for region shots in GNOME.
Delay should indeed happen before root and window screenshots, but must happen in-between region selection and region screenshot. One can notice that the logics is different for Windows screenshot in X11. The reason is that X11 window screenshot has an additional window selection step (and therefore delay must happen in between selection and actual screenshot). Window screenshot in Wayland doesn't have this step and simply screenshots whatever is the currently active Window.