ADDITIONAL SYSTEM INFORMATION :
Windows 10, Java 14.0.2

A DESCRIPTION OF THE PROBLEM :
In JavaFX 8 there was the possibility to change the user agent stylesheet of a Node at runtime and then call impl_reapplyCSS() or reapplyCss() with reflection and the control style would be updated.
In the next versions the behaviour has been changed, impl_reapplyCSS() has been removed because deprecated and if I call reapplyCss() with reflection the control's style is not updated anymore.
The only way to change the control style now is to force it by calling getStylesheets().setAll(STYLESHEET)
I think this regression may be related to JDK-8088253.
Please fix it as this should not be the intended behavior.

REGRESSION : Last worked in version 8u281

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Execute the first example with Java 8 and JavaFX 8 and then the second example with newer versions.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The control's style should update if the user agent stylesheet is updated
ACTUAL -
The CSS is not reloaded and the control's style is not updated.

---------- BEGIN SOURCE ----------
EXAMPLE 1
import custombutton.DynamicButton;
import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.Region;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

import static custombutton.DynamicButton.Styles.STYLE1;
import static custombutton.DynamicButton.Styles.STYLE2;

public class Test extends Application {
    @Override
    public void start(Stage primaryStage) throws Exception {
        StackPane pane = new StackPane();

        DynamicButton dynamicButton = new DynamicButton("TO CHANGE");
        Button button = new Button("CHANGE!");
        button.setOnAction(event -> {
            dynamicButton.setDynamicStyle(
                    dynamicButton.getDynamicStyle() == STYLE1 ? STYLE2 : STYLE1
            );
        });

        VBox box = new VBox(40, dynamicButton, button);
        box.setAlignment(Pos.TOP_CENTER);
        box.setMaxSize(Region.USE_PREF_SIZE, Region.USE_PREF_SIZE);
        pane.getChildren().add(box);

        primaryStage.setScene(new Scene(pane, 800, 600));
        primaryStage.show();
    }

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

package custombutton;

import javafx.beans.property.ObjectProperty;
import javafx.beans.property.SimpleObjectProperty;
import javafx.scene.control.Button;

public class DynamicButton extends Button {
    private final String STYLE_CLASS = "dynamic-button";
    private String STYLESHEET = getClass().getResource("dynamic1.css").toString();

    public static enum Styles {
        STYLE1,
        STYLE2
    }

    private final ObjectProperty<Styles> style = new SimpleObjectProperty<>(Styles.STYLE1);

    public DynamicButton() {
        initialize();
    }

    public DynamicButton(String text) {
        super(text);
        initialize();
    }

    private void initialize() {
        getStyleClass().add(STYLE_CLASS);

        style.addListener((observable, oldValue, newValue) -> {
            if (newValue == Styles.STYLE1) {
                STYLESHEET = getClass().getResource("dynamic1.css").toString();
            } else {
                STYLESHEET = getClass().getResource("dynamic2.css").toString();
            }

            impl_reapplyCSS();
            System.out.println("Style Changed");
        });
    }

    public Styles getDynamicStyle() {
        return style.get();
    }

    public ObjectProperty<Styles> dynamicStyleProperty() {
        return style;
    }

    public void setDynamicStyle(Styles style) {
        this.style.set(style);
    }

    @Override
    public String getUserAgentStylesheet() {
        System.out.println("getUserAgentStylesheet Called");
        return STYLESHEET;
    }
}


EXAMPLE 2
import custombutton.DynamicButton;
import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.Region;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

import static custombutton.DynamicButton.Styles.STYLE1;
import static custombutton.DynamicButton.Styles.STYLE2;

public class Test extends Application {
    @Override
    public void start(Stage primaryStage) throws Exception {
        StackPane pane = new StackPane();

        DynamicButton dynamicButton = new DynamicButton("TO CHANGE");
        Button button = new Button("CHANGE!");
        button.setOnAction(event -> {
            dynamicButton.setDynamicStyle(
                    dynamicButton.getDynamicStyle() == STYLE1 ? STYLE2 : STYLE1
            );
        });

        VBox box = new VBox(40, dynamicButton, button);
        box.setAlignment(Pos.TOP_CENTER);
        box.setMaxSize(Region.USE_PREF_SIZE, Region.USE_PREF_SIZE);
        pane.getChildren().add(box);

        primaryStage.setScene(new Scene(pane, 800, 600));
        primaryStage.show();
    }

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

package custombutton;

import javafx.beans.property.ObjectProperty;
import javafx.beans.property.SimpleObjectProperty;
import javafx.scene.Node;
import javafx.scene.control.Button;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class DynamicButton extends Button {
    private final String STYLE_CLASS = "dynamic-button";
    private String STYLESHEET = getClass().getResource("dynamic1.css").toString();

    public static enum Styles {
        STYLE1,
        STYLE2
    }

    private final ObjectProperty<Styles> style = new SimpleObjectProperty<>(Styles.STYLE1);

    public DynamicButton() {
        initialize();
    }

    public DynamicButton(String text) {
        super(text);
        initialize();
    }

    private void initialize() {
        getStyleClass().add(STYLE_CLASS);

        style.addListener((observable, oldValue, newValue) -> {
            if (newValue == Styles.STYLE1) {
                STYLESHEET = getClass().getResource("dynamic1.css").toString();
            } else {
                STYLESHEET = getClass().getResource("dynamic2.css").toString();
            }

            try {
                Method method = Node.class.getDeclaredMethod("reapplyCss");
                method.setAccessible(true);
                Object call = method.invoke(this);
            } catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
                e.printStackTrace();
            }
            System.out.println("Style Changed");
        });
    }

    public Styles getDynamicStyle() {
        return style.get();
    }

    public ObjectProperty<Styles> dynamicStyleProperty() {
        return style;
    }

    public void setDynamicStyle(Styles style) {
        this.style.set(style);
    }

    @Override
    public String getUserAgentStylesheet() {
        System.out.println("getUserAgentStylesheet Called");
        return STYLESHEET;
    }
}

CSS FILES
dynamic1.css
.dynamic-button {
    -fx-border-color: blue;
}

dynamic2.css
.dynamic-button {
    -fx-border-color: red;
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Since the CSS file is cached in STYLESHEET, the only way to update the control's style is to call getStylesheets().setAll(STYLESHEET);

FREQUENCY : always