MATLAB: ESP32 MQTT connecting and publishing to ThingSpeak but channel is not updating

esp32mqttpublishThingSpeak

I followed this example to use MQTT from my ESP32:
https://uk.mathworks.com/help/thingspeak/use-arduino-client-to-publish-to-a-channel.html
Lets make some fake variables for discussion:
long channelID = 1234567;
char mqttPass[] = "ABCDEFGHIJKLM"; // MQTT API KEY (generated from user profile)
char apiKey[] = "APIKEY"; //Channel Write API Key (generated from channel API keys tab)
I can connect with the example code (sometimes it fails, but it reconnects and then holds the connection well).
After the postingInterval has passed, the publish function is called:
void mqttPublishFeed(float field1, float field2, float field3, float field4, float field5, float field6, float field7, float field8) {
    
  // Create data string to send to ThingSpeak.
   String data = "field1=";
    data += String(field1, DEC);
    data += "&field2=";
    data += String(field2, DEC);
    data += "&field3=";
    data += String(field3, DEC);
    data += "&field4=";
    data += String(field4, DEC);
    data += "&field5=";
    data += String(field5, DEC);
    data += "&field6=";
    data += String(field6, DEC);
    data += "&field7=";
    data += String(field7, DEC);
    data += "&field8=";
    data += String(field8, DEC);
  
  int length = data.length();
  const char *msgBuffer;
  msgBuffer=data.c_str();
  Serial.println(msgBuffer);
  
  // Create a topic string and publish data to ThingSpeak channel feed. 
  String topicString = "channels/" + String( channelID ) + "/publish/"+String(apiKey);
  Serial.println(topicString);
  length = topicString.length();
  const char *topicBuffer;
  topicBuffer = topicString.c_str();
  mqttClient.publish( topicBuffer, msgBuffer );
  lastConnectionTime = millis();
}
Here is an example of the topicString and msgBuffer variables that are printed to the serial (using the fake variables state above)
msgBuffer:
field1=829.0000000000&field2=477.0000000000&field3=28.0499992371&field4=35.0566406250&field5=23.0000000000&field6=50.9949913025&field7=0.0000000000&field8=995.9716186523
topicString:
channels/1234567/publish/APIKEY
Problem:
I have confirmed that (on my serial) the channelID and the channel API write key are correct and data seems to publish without error (though it is QOS 0 so its really hard to tell). Despite this, my channel does not update. I have previously used HTTPS and had no issues with API keys or channel updating so I know they aren't the issue.

Best Answer

OK, as I suspected (in my comment). There was a limit being imposed on the payload. At first I thought it was related to the Free Account. However, after seeing similar limitations in HiveMQ, I realised it had to be a code related issue.
For others experiencing the same issues:
The PubSubClient library used in many of the documentation examples imposes as 256byte limit (which I imagine includes all of the headers, QOS details and timestamp etc. either way, if you don't specify your buffer size, it will go over and then not get sent. Since ThingSpeak requires QOS 0, you aren't going to get alerted to these issues, but they are there:
For the Adruino PubSubClient you must specify the packet size after the include:
#include <PubSubClient.h>
#define MQTT_MAX_PACKET_SIZE 2048
This allowed me to send 8 fields of decimal values.
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