GmailThread.getlables() fails randomly with the exception "Exception: Gmail operation not allowed."

This might be a weird one but I have some code that goes through my email and uses labels to trigger certain tasks. For example, if someone applies via Indeed, Indeed sends me an email. A filter will see this this email and apply the "Applications/indeedApplication" label. Then, a script that runs once per minute will find this label, and based on the label, perform some actions automatically.

This works perfectly about 99.97% of the time. The script runs with no issues and does the tasks, applies a completed label, and everything works as intended. 0.03% of the time, the task will fail, always at the same point, and always with the same exception:

Exception: Gmail operation not allowed.

It fails when the scrip tries to run the getLables() function on a GmmailThread (class of GmailApp API).

Code in question:

function addICandidate() {
  var label = GmailApp.getUserLabelByName("Applications/indeedApplication");   //Defines the label that identifies indeed applicatoins

  var requests = label.getThreads();                    //Returns an array of threads which have this label
  if(requests.length > 3){requests.length = 3};         //This makes it so the script doesn't waste time looking through every application ever.  

  for ( var i in requests ) {                           //Starting a normal for loop
    iLabels = requests[i].getLabels();                  //Gets the labels of the Randomly fails here and only here
    var checker = false;                                //used later to check if tasks were already completed
    for (var k = 0 in iLabels) {                        //Starting another normal for loop
      var individualLabel = iLabels[k].getName();       //Gets a list of the labels
      if (individualLabel === completedLabelName) {     //Checks if the label for completes tasks is present
        var checker = true;                             //If they are, set checker to true (also used later)
        }
      }
    if(checker){                                        //If the checker above is set to true
      Logger.log("true");                               //Do nothing (logger is just used in debugging)
    }
    else{                                               //Else (if the label is not completed and therefore, the tasks have not been done yet)
      var thread = requests[i].Dostuff();               //Confidential stuff lives here
 
      }
    requests[i].addLabel(completedlabel);               //After everything, apply the completed label so tasks are not done again on the next pass
    }
  }
}

Does anyone know why it would fail here and only 1 in 3000 attempts. I obviously cannot debug this with brute force so any ideas as to why this might happen would be really helpful.