Retrofit 2 streaming large JSON response

I'm trying to parse / process a JSON response while receiving (streaming / chunked). But I cannot get this to work. When I receive JSON I can only process it after the whole response has been received.

If I test the same code with a static file download, this is working correctly.

Retrofit: 2.6.1 OkHTTP 3.12.0

When I download the 5MB file the notice 'Call OK' will be shown immediately and the 'download complete' later on. For the JSON files the 'Call OK' will take a while.

Logger.info("Start download")
val response = fileApi.download5MBFile() // Streaming
val response = dataHubApi.download3MBJson() //Does not stream
val response = articleApi.download10MBJson() // Does not stream
if (response.isSuccessful) {
    Logger.info("Call ok")
    val input = response.body()?.byteStream()
    val buffer = ByteArray(8192)
    var size = 0

    while (true) {
        val read = input!!.read(buffer)

        if (read == -1) {
            break
        }
        size += read
        //Logger.info("Progress: ${size/1024/1024}mb")
    }
    Logger.info("Download complete")
} else {
    Logger.info("Call not ok")
}

The factory methods for creating the APIs are all the same like this:

    fun create(): FileApi {
        val logLevel = Level.HEADERS

        val retrofit = Retrofit.Builder()
            .baseUrl(URL)
            .client(getOkHttpClient(logLevel))
            .build()

        return retrofit.create(FileApi::class.java)
    }

    private fun getOkHttpClient(logLevel: Level): OkHttpClient {
        return OkHttpClient.Builder()
            .readTimeout(60L, TimeUnit.SECONDS) // default retrofit value is 10sec
            .build()
    }

And all the interface are also in the same format:

interface FileApi {
    @GET("/5MB.zip")
    @Streaming
    suspend fun download5MB(): Response<ResponseBody>
}

What can I do to also stream the JSON?