To get the fields to load on the front end you need to use an acf/load_field filter without the field key and you need to set the priority low.
add_filter('acf/load_value', 'add_starting_flex', 1, 3);
Then you need to test for the correct field key inside of your filter
if ($field['key'] != 'field_5eb8736d02e62' || $value !== NULL) {
return $value;
}
The reason that it is not working for you is that the filter using the field key runs too late and the flex field has a value other then NULL. The only way to bypass the built in ACF filter is by not specifying the field to filter.
