On page load and page save, dynamic defaults are evaluated and each NetSuite tag is substituted. For these substitutions to be made each time, the Store Value box for the custom field must be cleared.
If you check the Store Value box, the tag substitution is performed one time, when the custom field is created. The value is saved and does not change dynamically when the fields on the page are updated.
To ensure that NetSuite tags defined in a dynamic default are substituted every time a page is loaded and saved, clear the Store Value box.