Android BLE: readCharacteristic is not working

Question

I have a BLE peripheral running on Raspberry Pi and I'm trying to communicate with it via Android.

The goal is to read a characteristic, which is supposed to trigger a process on the peripheral side.

My code is as follows:

public class MainActivity extends AppCompatActivity
{
    static final String PERIPHERAL_NAME = "Gobbledego";
    static final String WIFI_SERVICE_UUID             = "B87CF566-7A73-4347-B711-BB20F8A23D08";
    static final String WIFI_SCAN_CHARACTERISTIC_UUID = "891FDFB1-3E77-4597-8483-4AB24ADC18CB";

    private BluetoothLeScanner scanner;
    private BluetoothGatt bluetoothGatt;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        scanner = BluetoothAdapter.getDefaultAdapter().getBluetoothLeScanner();
        scanner.startScan(scanCallback);
    }

    private ScanCallback scanCallback = new ScanCallback()
    {
        @Override
        public void onScanResult(int callbackType, ScanResult result)
        {
            super.onScanResult(callbackType, result);

            String name = result.getDevice().getName();
            if (name != null && name.equals(PERIPHERAL_NAME))
            {
                scanner.stopScan(scanCallback);
                bluetoothGatt = result.getDevice().connectGatt(MainActivity.this, false, gattCallback);
            }
        }
    };

    private final BluetoothGattCallback gattCallback = new BluetoothGattCallback()
    {
        @Override
        public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState)
        {
            if (newState == BluetoothProfile.STATE_CONNECTED)
            {
                bluetoothGatt.discoverServices();
            }
        }

        @Override
        public void onServicesDiscovered(BluetoothGatt gatt, int status)
        {
            if (status == BluetoothGatt.GATT_SUCCESS)
            {
                BluetoothGattService service = gatt.getService(UUID.fromString(WIFI_SERVICE_UUID));

                BluetoothGattCharacteristic wifiScanCharacteristic = service.getCharacteristic(UUID.fromString(WIFI_SCAN_CHARACTERISTIC_UUID));
                bluetoothGatt.readCharacteristic(wifiScanCharacteristic); // returns true
            }
        }
    };

The scanning part and the connection part are working. The problem is that the read is never acknowledged on the peripheral side. I know that the peripheral side is OK because I have an iOS based solution which works as intended.

I wonder what could be the problem?

EDIT: Tested on a Nexus 5 with Android 6.0.1