How to run JUnit5 Spring runner tests in parallel?

Question

I am unable to run JUnit5 tests concurrently when they involve using SpringExtension. When I run sequentially, there are no issues. When I run concurrently, only one test in the class is able to successfully access any injected or autowired fields. Any other tests will throw a NullPointerException when attempting to reference injected or autowired fields. Since JUnit5 parallelization is using ForkJoinPool under the hood, I was under the impression one Spring test context would be created and threads could safely use a cached context. Am I missing something in configuring my testing suite?

Here's a simplified example where the problem can be observed:

DummyTest:

import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.junit.jupiter.api.parallel.Execution;
import org.junit.jupiter.api.parallel.ExecutionMode;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit.jupiter.SpringExtension;
import javax.inject.Inject;

@Execution(ExecutionMode.CONCURRENT)
@ExtendWith(SpringExtension.class)
@ContextConfiguration(classes = {DummyTestConfig.class})
public class DummyTest {

    @Inject
    private StringBuilder stringBuilder;

    @Test
    public void testA() {
        Assertions.assertEquals(stringBuilder.toString(), "test");
    }

    @Test
    public void testB() {
        Assertions.assertEquals(stringBuilder.toString(), "test");
    }
}

DummyTestConfig:

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class DummyTestConfig {

    public DummyTestConfig() {}

    @Bean
    public StringBuilder stringBuilder() {
        return new StringBuilder("test");
    }

}