Usually, if it’s a new post then the post ID will be something like “new_post” or “new” or “new_???” depending on what you set the post ID for acf_form(). You should be checking for whatever you set there.
If you are using “new_post”, then this is a specific instance where ACF already has run it’s own pre_save_post filter. If you want to create your own pre_save_post filter then you should not use “new_post” as the post ID and use some other unique string. The reason is that ACF has already created the new post and a post ID has already been assigned to it.
