Is this for a new post? If it is, then there is no post id value. For new posts this value is created by ACF using a built in acf/pre_save_post filter or by a custom filter https://www.advancedcustomfields.com/resources/acf-pre_save_post/.
The only time that the post ID value will be set is for an existing post and that should be part of the submitted values.
