Google Spreadsheet Custom Functions when evaluated/executed?

Question

I got a Google Spreadsheet with two custom functions in a container-bound Appscript.

function helloWorld() {
  var message = "Hello World.";
  console.log(message);
  return message;
}

function getTimeStamp() {
  return new Date();
}

I'm then using them in some cells inside the Spreadsheet.

Then I changed the evaluation settings of the spreadsheet.

But my problem is, that the functions are not executed/evaluated as I thought. I don't see the updated timestamp and no new entry in the Stackdriver logs. So it's not being executed a) when opening the spreadsheet b) nor after the period that is specified in the properties. It only gets evaluated when I change the code in the script or when I add or remove rows/columns.

Can anyone tell me what the logic for the execution of custom functions is? I suggest that there is any caching in between. Is there any other way to execute all functions in a Spreadsheet like this pseudocode?

SpreadsheetApp.getActiveSheet().evaluate();