I've found some strange behaviour of TreeItem's graphics. Graphics disappears when scrolling is active and any node above collapses.
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Steps to reproduce:
resizing window or scrolling (except step 3) may frighten bug away
1)expand root
2)expand node 1 (or any from 0-3)
3)scroll down
you'll see this: http://i.imgur.com/z6VjIi9.png
4)expand node 4
5)collapse node 4
graphics of "leaf 3" disappeared:
http://i.imgur.com/OeLYOjY.png
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it seems to behave randomly from run to run, for example some combination of expanded/colapsed nodes makes graphics of "leaf 1"&"leaf 3" disappear.
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Here is my code for testing:

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.TreeItem;
import javafx.scene.control.TreeView;
import javafx.scene.image.Image;
import javafx.scene.image.ImageView;
import javafx.stage.Stage;

public class TreeTest extends Application {
    
    @Override
    public void start(Stage primaryStage) {
        Image img = new Image("http://i.imgur.com/XzPRH5e.png");//originally image was loaded from my hard drive
        TreeView<String> root = new TreeView<>();
        TreeItem<String> n0 = new TreeItem<>("root");
        root.setRoot(n0);
        for(int t=0;t<5;t++){
            TreeItem<String> n1 = new TreeItem<>("node "+t);
            n0.getChildren().add(n1);
            for(int i=0;i<5;i++){
                TreeItem<String> n2 = new TreeItem<>("node "+t+" "+i);
                n1.getChildren().add(n2);
                for(int j=0;j<3;j++){
                    TreeItem<String> n3 = new TreeItem<>("leaf "+t+" "+i+" "+j);
                    n3.setGraphic(new ImageView(img));
                    n2.getChildren().add(n3);
                }
            }
        }
        for(int j=0;j<4;j++){
            TreeItem<String> n1 = new TreeItem<>("leaf "+j);
            n1.setGraphic(new ImageView(img));
            n0.getChildren().add(n1);
        }
        Scene scene = new Scene(root, 300, 250);
        primaryStage.setTitle("Hello World!");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static void main(String[] args) {
        launch(args);
    }
    
}
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User can make graphics visible again by scrolling up and down for example, but this "magic" anyway is not acceptable.
Also I'm planning to use graphics to display states(no response/overloaded/overheated, etc.) and types of connected devices, so randomly disappearing graphics may be treated wrong.