To reproduce, run the example

- move thumb between two ticks 
- press button to enable snapToTicks
- expected: value (and with it the thumb) snapped to nearest tick
- actual: value/thumb unchanged, snapped at the next mouse touch only

Example:

public class SliderSnapToTickBug extends Application {

    @Override
    public void start(Stage primaryStage) {
        Slider slider = new Slider(5, 25, 15);
        // show ticks
        slider.setShowTickMarks(true);
        slider.setShowTickLabels(true);
        slider.setMajorTickUnit(10);
        
        Label valueLabel = new Label();
        valueLabel.textProperty().bind(slider.valueProperty().asString());
        
        Button snapTicks = new Button("snap to ticks");
        snapTicks.setOnAction(e -> {
            slider.setSnapToTicks(!slider.isSnapToTicks());
        });
        VBox root = new VBox(10, slider, valueLabel, snapTicks);
        primaryStage.setScene(new Scene(root, 500, 400));
        primaryStage.show();
    }
 
    public static void main(String[] args) {
        launch(args);
    }
    
}

Reason: SliderSkin doesn't listen to changes of the snapToTicks property
Fix: add listener and adjust value when changed to true

// in SliderSkin constructor
registerChangeListener(slider.snapToTicksProperty(), "SNAP_TO_TICKS");

// in handleControlProperty
} else if ("SNAP_TO_TICKS".equals(p)) {
    if (slider.isSnapToTicks()) {
        slider.adjustValue(slider.getValue());
    }
}

the fix is without any risk and well bounded - so could be added at once :-)