Do you have any other plugins on the site that use ACF? ACF only has one pre_save_post filter and it is only used if the post_id is “new_post”. Try deactivating other plugins or searching all the code on the site for “pre_save_post”.
Also, make sure you’re calling acf_form_head() before wp_get_header(). I don’t think this is it, but not calling acf_form_head() correctly can have some odd effects.
