TinyMQTT messages not fully delivered

I'm using TinyMQTT in a home automation project on a Pico + ESP8266.

About 20% of the time when handling new messages to subscribed topics, my message events fire but do not contain the topic or message, meaning I am unable to handle the message properly.

Subscribing using mosquitto_sub to check what is going on shows that the messages are definitely there and look normal, so this is an issue on the Pico side.

As a workaround I am sending an error back in the case of a null topic so that the UI can ask the user to try again but this is not satisfactory.

The fact it happens only a proportion of the time for identical messages suggests to me that it might be some kind of latency issue reading the data.

Anyone seen anything like this before with TinyMQTT? I am loth to switch to the much heavier MQTT, although I haven't tried it to see if it behaves better yet.

function MQTTmessage(msg) {
  console.log("MQTT message " + msg.topic + ":" + msg.message); 
  if (msg.topic.endsWith("/topic1")) {
    //doSomething();
  }
  if (msg.topic.endsWith("/topic2")) {
    //doSomethingElse();
  }
  if (msg.topic == "" || msg.topic == null) {
    //in this case msg.message is also empty
    throwError(null, "null message received - please try again", false);
  }