If you look at the following code, visually the text is exactly the same but one of them has a mnemonic. The one with the mnemonic is wider when it should be the same width.

import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.FlowPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class ButtonWidthTest extends Application {
    @Override
    public void start(Stage stage) throws Exception {
        FlowPane pane = new FlowPane();
        Scene scene = new Scene(pane, 500, 500);
        stage.setScene(scene);

        VBox vBox = new VBox();
        Button button1 = new Button("1:Fields");
        Button button2 = new Button("_1:Fields");
        button2.setMnemonicParsing(true);

        vBox.getChildren().addAll(button1, button2);

        pane.getChildren().add(vBox);
        stage.show();
    }

    public static void main(String[] args) {
        Application.launch(args);
    }
}