Test app below creates two stages one with a solid background scene and one with gradient. Both have some text with a drop shadow. The one with the gradient is all messed up, see attached pic.

import javafx.application.Application;
import javafx.geometry.Insets;
import javafx.scene.Scene;
import javafx.scene.effect.DropShadow;
import javafx.scene.layout.StackPaneBuilder;
import javafx.scene.paint.Color;
import javafx.scene.paint.CycleMethod;
import javafx.scene.paint.LinearGradient;
import javafx.scene.paint.Stop;
import javafx.scene.text.Font;
import javafx.scene.text.Text;
import javafx.stage.Stage;

/**
 * @author Jasper Potts
 */
public class EffectBug extends Application {

    public static void main(String[] args) {
        Application.launch(args);
    }
    
    @Override public void start(Stage primaryStage) {
        // BROKEN
        {
            Text label = new Text("Hello");
            label.setFont(Font.font("Arial Black", 72));
            label.setEffect(new DropShadow());
            Scene scene = new Scene(
                    StackPaneBuilder.create().children(label).padding(new Insets(20)).build(),
                    new LinearGradient(0, 0, 0, 1, true, CycleMethod.REPEAT, 
                        new Stop(0, Color.grayRgb(202)),
                        new Stop(1, Color.grayRgb(97))
                    ));
            primaryStage.setScene(scene);
            primaryStage.show();
        }
        
        // WORKING
        {
            Text label = new Text("Hello");
            label.setFont(Font.font("Arial Black", 72));
            label.setEffect(new DropShadow());
            Scene scene = new Scene(
                    StackPaneBuilder.create().children(label).padding(new Insets(20)).build(),
                    Color.YELLOW);
            Stage secondaryStage = new Stage();
            secondaryStage.setScene(scene);
            secondaryStage.show();
        }   
    }
}