There are few issues with your code:
1. As mentioned by @ACF Support, you should use $field_name in the get_field function.
2. Make sure you are loading from the correct post by adding in the $post_id parameter to the get_field function. This parameter is given to you in the acf/update_value filter.
3. Your update_field function is not targeting the user. You need to find the current user (with some WP code) and use that ID to target the save function.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users