can you share the code which is sending it? I think you need to place an if condition there if the selected key is ALL_MEMBEERS, then the selection is empty.
like this:
var selectionKey = DROPDOWN_1.getSelectedValue();
var realSelection = selectionKey;
if(selectionKey == "ALL_MEMBERS") { realSelection = "";
}
DS_1.setFilter...(<DIM>, realSelection");check up if this helps