Fail to process request with sub boundaries about wiremock HOT 1 OPEN

partBuilder.part("part2", subPartBuilder.build(), MULTIPART_MIXED);

    private Mono<String> callClient(WebClient webClient) {
        final var subPartBuilder = new MultipartBodyBuilder();
        subPartBuilder.part("part11", "part11_content").contentType(TEXT_PLAIN);
        subPartBuilder.part("part12", "part12_content").contentType(TEXT_PLAIN);

        final var partBuilder = new MultipartBodyBuilder();
        partBuilder.part("part1", "part1_content").contentType(TEXT_PLAIN);
        partBuilder.part("part2", subPartBuilder.build());

        return webClient
                .post()
                .contentType(MULTIPART_RELATED)
                .bodyValue(partBuilder.build())
                .retrieve()
                .bodyToMono(String.class);
    }