You need to create an action on the field, unfortunately I don’t know how to add an action specifically to a relationship field because of select2. To do this with a regular select field I would do something like
// field_XXXX is the field key of the field you want to target
$('[data-key="field_XXXX"] .acf-input select').on('change', function(e) {
// code to perform on change
});
NOt sure if this will help you.
