ADDITIONAL SYSTEM INFORMATION :
System:
Mac OS Mojave (MacBook Pro 11.5)
Version 10.14.6

Problem has been confirmed with OpenJDK 11.0.2 & OpenJFX 11.0.2 with both jpackage from JDK 14 ea and JDK 15 ea, and also with OpenJDK 14 ea & OpenJFX 14 ea.

EITHER:

openjdk version "11.0.2" 2019-01-15
OpenJDK Runtime Environment 18.9 (build 11.0.2+9)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.2+9, mixed mode)
&
OpenJFX 11.0.2 jmods from gluon
&
jpackage from 14-ea 2020-03-17 or openjdk 15-ea 2020-09-15

----
OR:

openjdk 14-ea 2020-03-17
OpenJDK Runtime Environment (build 14-ea+32-1423)
OpenJDK 64-Bit Server VM (build 14-ea+32-1423, mixed mode, sharing)
&
OpenJFX jmods 14 build 7 downloaded 10th Jan 2020.

A DESCRIPTION OF THE PROBLEM :
Custom resources do not get added to the app image on Mac OS X when using the --resources-dir flag. According to the documentation, this flag should be used when wanting to override the resources directory.

REGRESSION : Last worked in version 14

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Create a simple JavaFX project or use the example class provided, let's call it Main.java. Compile with javac providing path to OpenJFX mods in the --module-path and module javafx.controls with --add-modules.
2. Create jar: jar cfe myjar/Main.jar Main Main.class
3. Using jlink, create a custom JRE called myjre, including path to JFX in the --module-path and javafx.controls in the modules with --add-modules.
4. Create a directory ./myresources and add some file into it, such as a picture of your cat, cat.jpg.
5. Using jpackage from ea JDK 15, package the application:

jpackage --type app-image -n "My App" --main-class Main --main-jar Main.jar --runtime-image myjre --input myjar --resource-dir myresources --java-options "-verbose:class"

6. App image runs fine. However, inspecting the contents of the package, notice that the custom resources added with the --resource-dir flag didn't do anything. There is no cat.jpg in the Resources dir or anywhere else in the app contents.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Directory Contents/Resources in the app contents should contain custom resources from the resources directory provided with the --resources-dir flag. According to documentation, the flag is used for the following: "Path to override jpackage resources Icons, template files, and other resources of jpackage can be over-ridden by adding replacement resources to this directory. (absolute path or relative to the current directory)"
ACTUAL -
Nothing happens. The Contents/Resources dir only contains the default icon, indicating that the directory wasn't overriden. My guess is that it is getting overwritten again when the icon is set.

---------- BEGIN SOURCE ----------
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;

public class Main extends Application {

    @Override
    public void start(Stage stage) {
        String javaVersion = System.getProperty("java.version");
        String javafxVersion = System.getProperty("javafx.version");
        Label l = new Label("JavaFX " + javafxVersion + ", running on Java " + javaVersion + ".");
        Scene scene = new Scene(new StackPane(l), 640, 480);
        stage.setScene(scene);
        stage.show();
    }

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

}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
None known.

FREQUENCY : always