acf/update_value happens as each field is updated. Fields are updated in the order they are submitted, which is the same order that they appear in the HTML. If you attempt to update a field that is processed after the field you add the filter to then ACF will update that field with the submitted value overwriting what you updated the field to.
In this case you would be better off using an acf/save_post filter with a priority > 10 and do your updated then.
