Maybe this helps: I tried to dig through this problem and in my case I create post using acf/pre_save_post. I save post as a draft. The thing is all the metadata is saved for draft post, but when I try to see preview, WordPress actually shows me the first revision, but that revision does not have any metadata attached unless I open post in admin panel and press ‘Save Draft’ or keep it open long enough to trigger auto-save. It seems that WordPress’ hook wp_insert_post does not handle creating metadata for first revision.
