Subscription callback fired when no messages sent. data has random strings.

[rickblair]

• Arduino board: Adafruit HUZZA 32 feather

• Arduino IDE version (found in Arduino -> About Arduino menu): 1.8.1

• List the steps to reproduce the problem below (if possible attach a sketch or
  copy the sketch code in too):

I am using AdafruitIO
Here is the callback:
void onoffcallback(char *data, uint16_t len) {

Serial.print("Hey we're in a onoff callback, the button len -- value is: ");
Serial.print(len);
Serial.print(" -- ");
Serial.println(data);
String input = String(data);
if(oldAlert.equals(input))
{

Serial.println("No Change Punt");

return;

}
oldAlert = input;
if(input.startsWith("ON"))
{

String msg = "BARCODE: ";
hour += 1;
String hr = String(hour);
msg.concat(hr);
;
Serial.print("publishing: ");
Serial.println(msg);
barcodePub.publish(msg.c_str());

showAlert = true;
}
else
{
showAlert = false;
}
}

Here is the subscription code:

Adafruit_MQTT_Subscribe onoff = Adafruit_MQTT_Subscribe(&mqtt, AIO_USERNAME "/feeds/alert",MQTT_QOS_1); //Note tried all different QOS settings No change.

Here is loop:

MQTT_connect();
//
onoffPub.publish(oldAlert.c_str());
while(1)
{
// Ensure the connection to the MQTT server is alive (this will make the first
// connection and automatically reconnect when disconnected). See the MQTT_connect
// function definition further below.
MQTT_connect();

// wait 10 seconds for subscription messages
// since we have no other tasks in this example.
mqtt.processPackets(10000);

// keep the connection alive
mqtt.ping();
}
}

Every 10 seconds or a multiple of 10 seconds I get this output.

Hey we're in a onoff callback, the button len -- value is: 99 -- OFFARCODE: 1aa5da0357e4785b49b0202463d236c

It should just be either OFF or ON

I publish an ON message which shows up as expected:
Hey we're in a onoff callback, the button len -- value is: 2 -- ON

Then an off message as expected:
Hey we're in a onoff callback, the button len -- value is: 3 -- OFF

Then after about 20 secs I start getting:
Hey we're in a onoff callback, the button len -- value is: 99 -- OFFARCODE: 2aa5da0357e4785b49b0202463d236c

I do have a barcode topic:
Adafruit_MQTT_Publish barcodePub = Adafruit_MQTT_Publish(&mqtt, AIO_USERNAME "/feeds/barcode");

Where I publish a message in the callback. Somehow the string I use in the barcode is being used in the buffer in the subscribe code?

I can work around this, but it is disturbing to see random data showing up in a phantom subscription.

[rickblair]

A bit more info:
I am running the code as a task pinned to a core:
xTaskCreatePinnedToCore(
codeForMQTT,
"MQTTTask",
5000,
NULL,
1,
&padTask,
1);

That is why there is a while(1) in the loop code above. I got the same results when the code was running in loop instead of a pinned task.