Did you mean to mark my question as the answer? Sorry, I was wondering about the field names because field names cannot start with ‘field_’ and if they did that could have been a cause of the issue.
For JSON values you could use either a text field or a textarea field with not formatting. Or you could use any type of field that will hold text data and when getting the value set the 3rd parameter in get_field($field, $post_id, $format_value) to false to make sure the value is not formatted or altered in any way.
