How to add to List in nested flatMaps using different publishers with Reactor - project-reactor

Question

I'm new to Reactor and I would like to know what is the correct way to implement this simple logic. I want to compare two Collections of items and if items are matched I want to add them to Mono<List>. Items are matched if they are equal or if item is parent of other item.
My test method sometimes passes and other times fails with incorrect number of items in Mono<List>. I can't figure out what is the problem. It seems that when I run the junit tests for class with this single test method it passes. However, when I put other test methods, to test different inputs/outputs, in the same class then it sometimes fails with incorrect items in the list. As if the test method finished sooner, before the list got updated with all added items.
package test;
import static org.assertj.core.api.Assertions.assertThat;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import org.junit.Test;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
import reactor.test.StepVerifier;
public class ReactorBasicsTests {
#Test
public void testMatching() {
Mono<List<String>> matched = matching(
Arrays.asList("fruit", "vegetable", "meat"),
Arrays.asList("apple", "orange", "carrot", "meat"));
StepVerifier.create(matched)
.consumeNextWith(list -> {
assertThat(list).containsExactlyInAnyOrder(
"fruit+apple", "fruit+orange", "vegetable+carrot", "meat+meat");
})
.expectComplete()
.verify();
}
/**
* Items are matched when:
* 1) they are equal (meat equals to meat)
* 2) item is parent of child item (fruit is parent of apple)
*/
public Mono<List<String>> matching(List<String> list1, List<String> list2) {
Flux<String> items1 = Flux.fromIterable(list1);
Flux<String> items2 = Flux.fromIterable(list2);
List<String> matched = new ArrayList<>();
return items1
.flatMap(item1 -> items2
.flatMap(item2 -> {
// Check if item1 is equal to item2
Mono<Void> isEqual = Mono.fromSupplier(() -> {
if (item1.equals(item2)) {
matched.add(item1 + "+" + item2);
}
return null; // I have to return something to compile the code
}).then();
// Check if item1 is parent of item2
Mono<Void> isParent = this.getParents(item2).hasElement(item1)
.flatMap(booleanValue -> {
if (booleanValue) {
matched.add(item1 + "+" + item2);
}
return Mono.empty();
});
return isEqual.then(isParent).then();
})
).then(Flux.fromIterable(matched).collectList());
}
/** This method simulates reactive database repository request that returns Flux<String> **/
public Flux<String> getParents(String item) {
if (item.equals("apple") || item.equals("orange")) return Flux.just("fruit", "food");
else if (item.equals("carrot")) return Flux.just("vegetable", "food");
return Flux.empty();
}
}