After reviewing @hugronaphor's answer. In particular this link along with some jQuery 3.x discussions regarding changes to the ready and load events my final solution was in fact remove my code from Drupal.behaviours.

In the end I have implemented code in the following style:

(function ($, Drupal){
    $(window).on("load",function() {
        //Do something
    });
})(jQuery, Drupal);

This gave me reliable access to the .on("load") event again which is what I was after as I need the entire page to be loaded before I trigger the script.

As my script is relatively simple and doesn't need further context information I think this is fine for my use case.

I "think" the issue is related to the following change in jQuery 3.x where on("load") is not guaranteed to fire inside a $(Document).ready state any more - which is what Drupal.behaviours is giving you. I am something of a js noob though so will leave it to smarter people to explain why this may or may not be the case. If there is a better answer I would like to hear it.

For my simple case not being in the Drupal.behaviour has solved my issue.

After reviewing @hugronaphor's answer. In particular this link along with some jQuery 3.x discussions regarding changes to the ready and load events my final solution was in fact to remove my code from Drupal.behaviours.

In the end I have implemented code in the following style:

(function ($, Drupal){
    $(window).on("load",function() {
        //Do something
    });
})(jQuery, Drupal);

This gave me reliable access to the .on("load") event again which is what I was after as I need the entire page to be loaded before I trigger the script.

As my script is relatively simple and doesn't need further context information I think this is fine for my use case.

I "think" the issue is related to the following change in jQuery 3.x where on("load") is not guaranteed to fire inside a $(Document).ready state any more - which is what Drupal.behaviours is giving you. I am something of a js noob though so will leave it to smarter people to explain why this may or may not be the case. If there is a better answer I would like to hear it.

For my simple case not being in the Drupal.behaviour has solved my issue.