ServiceLoader does not load implementation jars

Question

I am trying to implement a simple modular application with a self written plugin loader. For that I created a gradle multiproject build. The main project is going to be the started application. In the subproject spi is a module containing the Plugin interface, while in the subproject simple-plugin I wanted to create a plugin to test the plugin loading.

The Plugin interface is very simple:

package io.github.zeroneca.orgascreen.spi;

public interface Plugin {
    String getId();
}

module-info.java:

module io.github.zeroneca.orgascreen.spi {
    exports io.github.zeroneca.orgascreen.spi;
}

SimplePlugin.java:

package io.github.zeroneca.orgascreen.plugins.simple;

import io.github.zeroneca.orgascreen.spi.Plugin;

public class SimplePlugin implements Plugin {
    private final String id = "SimplePlugin";

    @Override
    public String getId() { return id; }
}

module-info.java

module io.github.zeroneca.orgascreen.plugins.simple {
    requires io.github.zeroneca.orgascreen.spi;
    provides io.github.zeroneca.orgascreen.spi.Plugin with io.github.zeroneca.orgascreen.plugins.simple.SimplePlugin;
}

Now in the class PluginLoader I'm loading all jars from a directory and want to add all Plugins to a List:

package io.github.zeroneca.orgascreen;

// several imports

public class PluginLoader {
    private static final List<Plugin> PLUGINS = new ArrayList<>();

    public static void load() {
        URL[] pluginURLs = getPluginURLs();  // returns all URLs of jar files in a directory
        URLClassLoader urlClassLoader = URLClassLoader.newInstance(pluginURLs);
        ServiceLoader<Plugin> serviceLoader = ServiceLoader.load(Plugin.class, urlClassLoader);
        for (Plugin plugin : serviceLoader) {
            PLUGINS.add(plugin);
        }
    }
    // some more methods hidden here
}

module-info.java

module io.github.zeroneca.orgascreen {
    requires io.github.zeroneca.orgascreen.spi;
    uses io.github.zeroneca.orgascreen.spi.Plugin;
}

Now after building with gradle and copying the resulting jar simple-plugin-0.1.jar into the directory, I can see, that the URL of the file is in pluginJars but the List PLUGINS remains empty. I'm not sure what I'm doing wrong, online searches didn't help, I cannot see a difference to working solution, but the use of gradle.

gradle 6.5.1 is in use and modularity.inferModulePath is set to true.

Help is greatly appreciated!

Note: when adding META-INF/services/io.github.zeroneca.orgascreen.spi.Plugin to the jar of SimplePlugin it works, so something must be wrong with the module declarations. But I just can't see what.