How to fire jQuery after $(window).on(‘load’)

It likely does not have to do with WordPress Core – in most cases this type of issue comes about when plugins conflict. I would suggest deactivating all your other plugins and trying a default theme, a la Twenty Seventeen. If it works at that point, you can switch the theme & test, then activate one plugin at a time & test, until you find what’s conflicting, then report that to the plugin author so they can find a workaround.

In the meantime, if using a default theme and only this plugin still isn’t working, here are two things to try this in place of $(window).on('load'):

jQuery(window).load(function($) {
   // your function
});

Or more succinctly

(function($) {
  // your function
})(jQuery);

WP uses jQuery no conflict – so it could simply be the $ syntax. If that fixes it, it’s often fastest to submit a pull request to the plugin repo to spur the author into action.