You’re trying to set the default value. The default value is only used on new posts. When this is first run on a new post, both of the other fields will have no value so you’re setting the default value to nothing. After the post is saved the first time the default value is ignored.
If you want this field set based on the other fields then I would suggest an acf/save_post action that updates this field when the post is saved, https://www.advancedcustomfields.com/resources/acf-save_post/, https://www.advancedcustomfields.com/resources/update_field/
