Here is a reproduced test case from async profiler:
https://github.com/async-profiler/async-profiler/pull/981

Both G1 and ZGC crash. The stack traces are G1/ZGC concurrent marking or G1 full gc marking.

Main.java:
===========================================================
import java.util.Base64;

public class Main extends Thread {
    public static void main(String[] args) throws Exception {
        long last = System.nanoTime();
        for (int i = 0;; i++) {
            CustomClassLoader loader = new CustomClassLoader();
            Class<?> k = loader.findClass("TemplateFFFFFFFF");
            Object o = k.getDeclaredConstructor().newInstance();

            // call gc every ~1 second.
            if ((System.nanoTime() - last) >= 1e9) {
                System.gc();
                last = System.nanoTime();
            }
        }
    }
}

class CustomClassLoader extends ClassLoader {
    @Override
    public Class findClass(String name) throws ClassNotFoundException {
        /*
         * Bytecode for:
         * public class TemplateFFFFFFFF {
         * public void doTemplateFFFFFFFF() {
         * return;
         * }
         * }
         */
        byte[] b = Base64.getDecoder()
                .decode("yv66vgAAADQADgoAAwALBwAMBwANAQAGPGluaXQ+AQADKClWAQAEQ29kZQEAD0xpbmVOdW1iZXJU" +
                        "YWJsZQEAEmRvVGVtcGxhdGVGRkZGRkZGRgEAClNvdXJjZUZpbGUBABVUZW1wbGF0ZUZGRkZGRkZG" +
                        "LmphdmEMAAQABQEAEFRlbXBsYXRlRkZGRkZGRkYBABBqYXZhL2xhbmcvT2JqZWN0ACEAAgADAAAA" +
                        "AAACAAEABAAFAAEABgAAAB0AAQABAAAABSq3AAGxAAAAAQAHAAAABgABAAAAAQABAAgABQABAAYA" +
                        "AAAZAAAAAQAAAAGxAAAAAQAHAAAABgABAAAAAwABAAkAAAACAAo=");
        return defineClass(name, b, 0, b.length);
    }
}
=========================================================

repro.cpp:
=========================================================
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <jvmti.h>
#include <jni.h>
#include <pthread.h>

static jvmtiEnv *jvmti;
static JavaVM *_jvm;
static JNIEnv *_rb_env;

#ifndef WITH_GetClassSignature
#define WITH_GetClassSignature 1
#endif

#ifndef WITH_DeleteLocalRef
#define WITH_DeleteLocalRef 0
#endif

#define BUFFER_SIZE 100000
static size_t ring_buffer[BUFFER_SIZE] = {0};
static volatile int ring_buffer_idx = 0;
static int reader_created = 0;

void *get_method_details(void *arg)
{
    jmethodID method = (jmethodID)arg;

    jclass method_class;
    char *class_name = NULL;

    jvmtiError err = JVMTI_ERROR_NONE;

    // For JVM 17, 21, 22 calling GetMethodDeclaringClass is enough.
    if ((err = jvmti->GetMethodDeclaringClass(method, &method_class)) == 0)
    {
        if (WITH_DeleteLocalRef)
        {
            _rb_env->DeleteLocalRef(method_class);
        }

        if (WITH_GetClassSignature)
        {
            // JVM 8 needs this to crash
            jvmti->GetClassSignature(method_class, &class_name, NULL);
            jvmti->Deallocate((unsigned char *)class_name);
        }
    }
}

void *read_ringbuffer(void *arg)
{
    JNIEnv *env;
    _jvm->AttachCurrentThread((void **)&env, NULL);
    _rb_env = env;

    for (;;)
    {
        size_t id = ring_buffer[rand() % BUFFER_SIZE];
        if (id > 0)
        {
            get_method_details((void *)id);
        }
    }
}

static void JNICALL ClassPrepareCallback(jvmtiEnv *jvmti_env,
                                         JNIEnv *jni_env,
                                         jthread thread,
                                         jclass klass)
{
    if (reader_created == 0)
    {
        pthread_t tid;
        pthread_create(&tid, NULL, read_ringbuffer, NULL);

        reader_created = 1;
    }

    // Get the list of methods
    jint method_count;
    jmethodID *methods;
    if (jvmti_env->GetClassMethods(klass, &method_count, &methods) == JVMTI_ERROR_NONE)
    {
        for (int i = 0; i < method_count; i++)
        {
            ring_buffer[ring_buffer_idx++] = (size_t)methods[i];
            ring_buffer_idx = ring_buffer_idx % BUFFER_SIZE;
        }
        jvmti_env->Deallocate((unsigned char *)methods);
    }
}

JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *jvm, char *options, void *reserved)
{
    jvmtiEventCallbacks callbacks;
    jvmtiError error;

    _jvm = jvm;

    if (jvm->GetEnv((void **)&jvmti, JVMTI_VERSION_1_0) != JNI_OK)
    {
        fprintf(stderr, "Unable to access JVMTI!\n");
        return JNI_ERR;
    }

    // Set up the event callbacks
    memset(&callbacks, 0, sizeof(callbacks));
    callbacks.ClassPrepare = &ClassPrepareCallback;

    // Register the callbacks
    error = jvmti->SetEventCallbacks(&callbacks, sizeof(callbacks));
    if (error != JVMTI_ERROR_NONE)
    {
        fprintf(stderr, "Error setting event callbacks: %d\n", error);
        return JNI_ERR;
    }

    // Enable the ClassPrepare event
    error = jvmti->SetEventNotificationMode(JVMTI_ENABLE, JVMTI_EVENT_CLASS_PREPARE, NULL);
    if (error != JVMTI_ERROR_NONE)
    {
        fprintf(stderr, "Error enabling ClassPrepare event: %d\n", error);
        return JNI_ERR;
    }

    return JNI_OK;
}
=============================================================

Steps to reproduce:

javac Main.java
gcc -shared -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/linux" -fPIC repro.cpp -orepro.so

# Low Xmx to pressure GC into unloading classes sooner.
java -agentpath:"$(pwd)/repro.so" -Xmx100m Main