I don’t think any of this is still relevant with the new ACF JS API and I don’t know how to replace .fetch(). If I remember correctly update.fetch() was a method in acf.ajax which no longer exists. What you’d have to do is find the method in the ACF scripts that is used to update the field groups on the page and see if you can call it.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users