I’m not completely familiar with the acf js api in this case.
I would first figure out if the “on” function is actually being called
$field.on("change", function(e) {
console.log('my function was called');
Then I would figure out if the AJAX request is working.
Then, using the code you are using you will need to pass the current field to the function that updates the other field so that you can find that field. The field shold be in “e” somewhere in this case.
Unfortunately I also do not know how to use the ACF JS API to find sibling fields in a repeater, or even if it is possible. I have a tendency to side-step the API.
