The problem is most likely that during the post process the acf field key reference is not being saved. For every custom field saved to the db there is also a reference field. The meta_key for this is '_'.{$field_name} and the value of this field is the ACF field key. Without this reference ACF does not know what to do with the field value when you use an function like get_field($field_name);
There are a few choices.
The first is to use a WP filter and somehow create this ACF field key reference when the post is added.
The second is to use the acf field key instead of the field name when calling update_field()
The 3rd is to use the field key when getting the field in your template when using and acf function like get_field()
and the last you already know is to update the post manually.
