How to execute code that uses reflection during build in a maven project?

I have a maven project in which I try to execute code during build (ideally between compile and test phases, but I'm not there yet, for now I execute the code after goals execution).

For this purpose, I use :

<groupId>org.codehaus.mojo</groupId>
<artifactId>exec-maven-plugin</artifactId>

My code is executed during build. However, it uses com.google.guava reflection, which does not seem to work as expected in this context.

Here is a MRE code :

package com.example;

import com.google.common.reflect.ClassPath;

import java.io.IOException;

public class LoadedClassesPrinter {

    public static void main(String[] args) throws IOException {
        ClassPath.from(ClassLoader.getSystemClassLoader())
                .getAllClasses()
                .stream()
                .forEach(o -> System.out.println(o.getName()));
    }

}

And here is a MRE pom.xml :

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>maven-exec-reflection</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>16</maven.compiler.source>
        <maven.compiler.target>16</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>com.google.guava</groupId>
            <artifactId>guava</artifactId>
            <version>31.1-jre</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>3.0.0</version>
                <configuration>
                    <mainClass>com.example.LoadedClassesPrinter</mainClass>
                </configuration>
            </plugin>
            <plugin>
                <artifactId>maven-assembly-plugin</artifactId>
                <version>3.3.0</version>
                <configuration>
                    <archive>
                        <manifest>
                            <mainClass>com.example.LoadedClassesPrinter</mainClass>
                        </manifest>
                    </archive>
                    <descriptorRefs>
                        <descriptorRef>jar-with-dependencies</descriptorRef>
                    </descriptorRefs>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

When I execute the main class from my IDE or from the generated jar (with dependencies), it works : it prints a bunch of classes, including my LoadedClassesPrinter, as I expect.

...
com.google.j2objc.annotations.RetainedLocalRef
com.google.j2objc.annotations.RetainedWith
com.google.j2objc.annotations.Weak
com.google.j2objc.annotations.WeakOuter
com.example.LoadedClassesPrinter

But when I run mvn clean install assembly:single exec:java, the main method is executed, but it only prints a much smaller list of classes, which, to my surprise, does not include my LoadedClassesPrinter:

org.codehaus.classworlds.BytesURLConnection
org.codehaus.classworlds.BytesURLStreamHandler
org.codehaus.classworlds.ClassRealm
org.codehaus.classworlds.ClassRealmAdapter
org.codehaus.classworlds.ClassRealmReverseAdapter
org.codehaus.classworlds.ClassWorld
org.codehaus.classworlds.ClassWorldAdapter
org.codehaus.classworlds.ClassWorldReverseAdapter
org.codehaus.classworlds.ConfigurationException
org.codehaus.classworlds.Configurator
org.codehaus.classworlds.ConfiguratorAdapter
org.codehaus.classworlds.DefaultClassRealm
org.codehaus.classworlds.DuplicateRealmException
org.codehaus.classworlds.Launcher
org.codehaus.classworlds.NoSuchRealmException
org.codehaus.plexus.classworlds.ClassWorld
org.codehaus.plexus.classworlds.ClassWorldException
org.codehaus.plexus.classworlds.ClassWorldListener
org.codehaus.plexus.classworlds.UrlUtils
org.codehaus.plexus.classworlds.launcher.ConfigurationException
org.codehaus.plexus.classworlds.launcher.ConfigurationHandler
org.codehaus.plexus.classworlds.launcher.ConfigurationParser$1
org.codehaus.plexus.classworlds.launcher.ConfigurationParser
org.codehaus.plexus.classworlds.launcher.Configurator$1
org.codehaus.plexus.classworlds.launcher.Configurator
org.codehaus.plexus.classworlds.launcher.Launcher
org.codehaus.plexus.classworlds.realm.ClassRealm
org.codehaus.plexus.classworlds.realm.DuplicateRealmException
org.codehaus.plexus.classworlds.realm.Entry
org.codehaus.plexus.classworlds.realm.NoSuchRealmException
org.codehaus.plexus.classworlds.strategy.AbstractStrategy
org.codehaus.plexus.classworlds.strategy.OsgiBundleStrategy
org.codehaus.plexus.classworlds.strategy.ParentFirstStrategy
org.codehaus.plexus.classworlds.strategy.SelfFirstStrategy
org.codehaus.plexus.classworlds.strategy.Strategy
org.codehaus.plexus.classworlds.strategy.StrategyFactory

I found a set of configuration for exec-maven-plugin and I tried to use them but it never changed anything.

What am I doing wrong? Is there an easy way to execute code to list my classes using reflection during build? It does not have to be exec-maven-plugin and com.google.guava, I am open to other plugins and libraries.

(I use maven 3.8.3 and java 16)