This code snippet displays the default value if the field value is empty in the profile view mode. You can add this code to the functions.php file in the active theme directory.

um_field_value_show_default.php

<?php

/**
 * Display default value if the field value is empty in the profile view mode.
 *
 * @param  string $value   Field Value.
 * @param  string $default Field Default Value.
 * @param  string $key     Field Key.
 * @param  string $type    Field Type.
 * @param  array  $data    Field Data.
 * @return string          Field value to display.
 */
function um_field_value_show_default( $value, $default, $key, $type, $data ) {
	if ( empty( $value ) && ! empty( UM()->fields()->viewing ) ) {
		if ( ! empty( $default ) ) {
			$value = $default;
		}
	}
	return $value;
}
add_filter( 'um_field_value', 'um_field_value_show_default', 50, 5 );

This gist is a part of the article How to display fallback value if the field value is empty.

Documentation: https://docs.ultimatemember.com/
Support forum: https://wordpress.org/support/plugin/ultimate-member/