This is confusing: calls to get_fields() find no Advanced Custom Fields values saved with a specific post, but calls to get_field() find values for the same post.
This happens when ACF did not insert the field values, and the secondary value with the meta_key beginning with an underscore does not exist.
The post has the values, but the Advanced Custom Fields plugin expects an additional value in a hidden field for each field.
ACF Creates a Second Key Value Pair
my_custom_value_my_custom_value(this one is missing)