HttpUrlConnection hangs out on POST request

Question

I am working on custom gradle plugin which trigger API POST call. There is no many option for networking from gradle and UrlConnection looks as a best tool. I am able to trigger GET API call, but can not do it for POST.

Here is my client code from gradle plugin:

interface SwapTestPluginExtension {
    val backEndEndpoint: Property<String>
}

class SwapTestPlugin: Plugin<Project> {
    override fun apply(target: Project) {

        val extension = target.extensions.create("swapTest", SwapTestPluginExtension::class.java)
        target.task("prepareTestEnvironment") {
            doLast {
                try {
                    println("[start] ${SwapTestPlugin::class.simpleName}")
                    executePostApiCall(extension)
                } catch (ex: Exception) {
                    println("Something whent wrong in ${SwapTestPlugin::class.simpleName} with exception ${ex}")
                } finally {
                    println("[end] ${SwapTestPlugin::class.simpleName}")
                }
            }
        }
    }

    private fun executePostApiCall(extension: SwapTestPluginExtension) {
        val credentials = "{\"name\": \"\", \"pwd\" : \"\"}"
        val req = URL("${extension.backEndEndpoint.get()}/test/environment").openConnection() as HttpURLConnection
        req.setRequestMethod("POST")
        req.setRequestProperty("Content-Type", "application/json; charset=UTF-8")
        req.setRequestProperty("Accept", "*/*");
        req.setDoOutput(true)
        println("[swap-test-plugin] prepare to call API")
        req.getOutputStream().write(JsonOutput.toJson(credentials).toByteArray(Charsets.UTF_8)/*.getBytes("UTF-8")*/)
        req.outputStream.flush()
        println("[swap-test-plugin] API has been called, going to read results")
        val br = BufferedReader(InputStreamReader(req.inputStream, Charsets.UTF_8.name()))
        val response = StringBuilder()
        var responseLine: String? = null
        while (br.readLine().also { responseLine = it } != null) {
            response.append(responseLine!!.trim { it <= ' ' })
        }
        println("[swap-test-plugin] prepare to parse result")
        val json = JsonSlurper().parseText(response.toString())
        println("[json] ${json}")
    }

}

Here is stubs for API methods on the backend:

controllers/test.js

const config = require('config');
const network = require('../utils/network')
const logger = require('../utils/logger')

exports.setEnvironment = async function(req, res) { 
    logger.log("[start] /test/environment call");
    let result
    if (config.isProduction) {
        result = network.getErrorMsg(501, "This method is only supported for non-production mode for test purposes.")
    } else {
        // TODO clean up database if it is a debug mode
        result = network.getMsg(200, JSON.stringify({"msg" : "just a stub"}));
    }
    logger.log(`[end] response ${JSON.stringify(result)}`)
    network.send(req, res, result);
}

routes/index.js
...
router.post('/api/v1/test/environment', tests.setEnvironment);
...

Wireshark shows an POST request happens, but there is not activity on the server - it looks like it never reaches. The similar GET request works fine. Here is a source code of the backend in case you would like to copy-paste this block of code and reproduce.

1589    960.149964545   127.0.0.1   127.0.0.1   HTTP/JSON   100 POST /api/v1/test/environment HTTP/1.1 , JavaScript Object Notation (application/json)
1590    960.149967554   127.0.0.1   127.0.0.1   TCP 66  3100 → 50170 [ACK] Seq=1 Ack=262 Win=65280 Len=0 TSval=3277455079 TSecr=3277455079

Do you notice cause of this issue?