HiveMQ client fails authentication

Question

I'm working on android app where I need to communicate over MQTT, so for this one I've chose HiveMQ. The thing is that sometimes it will connect straight away, but there are cases when I'll receive a lot of errors saying NOT_AUTHORIZED or it won't connect at all.

Do you have any suggestions or see something wrong?

This is my code for the builder:

Mqtt5Client.builder()
        .identifier(deviceSerialNo)
        .serverHost(SERVER_HOST)
        .serverPort(SERVER_PORT)
        .automaticReconnect(
            MqttClientAutoReconnect.builder()
                .initialDelay(2L, TimeUnit.SECONDS)
                .maxDelay(10L, TimeUnit.SECONDS)
                .build()
        )
        .sslWithDefaultConfig()
        .addConnectedListener {
            Log.d(TAG, "Client: ${client.state}")
        }
        .addDisconnectedListener {
            Log.d(TAG, "Client: DISCONNECTED => Reason: ${it.cause.message}")
        }
        .build()

Connect method:

client.toBlocking().connectWith()
     .simpleAuth()
     .username("")
     .password(
          generateAuthPassword(deviceSerialNo, credentials).toByteArray()
     )
     .applySimpleAuth()
     .send()

private fun generateAuthPassword(
        serial: String,
        credentials: String
    ): String {
        val expiryDate = DocSigningHelper.generateExpiryDate()
        val uriDoc = "URI_DOC?sx=$expiryDate"
        val signature = DocSigningHelper.generateSignature(uriDoc, credentials)
        return "$uriDoc&si=$signature"
    }

object DocSigningHelper {
    fun generateExpiryDate(): Long =
        (System.currentTimeMillis() + 100_000L) / 1000L

    @OptIn(ExperimentalEncodingApi::class)
    fun generateSignature(doc: String, credentials: String): String {
        val decodedUsername = Base64.decode(credentials)
        val macAlgorithm = "HMACSHA256"
        val hashMethod = Mac.getInstance(macAlgorithm)
        val secretKey = SecretKeySpec(decodedUsername, macAlgorithm)
        hashMethod.init(secretKey)
        val signature = hashMethod.doFinal(doc.toByteArray(Charsets.UTF_8))

        return Base64.encode(signature)
    }
}