According to http://www.w3schools.com/cssref/css3_pr_border-image-slice.asp border-image-slice have 100% as default value.
According to JavaFX CSS Reference (http://docs.oracle.com/javafx/2/api/javafx/scene/doc-files/cssref.html) -fx-border-image-slice has null as default value and it is converted to 0 (0%).

So, instead of 100% sliced image we can see no image at all. It is unexpected behavior for ordinary developer.