Searching by anything other than post title/content in WP is difficult. ACF basically uses the built in WP admin search the same way that WP uses it when, for example you search for posts on the list of posts. You can’t just add the post ID into the s argument and expect this to work.
I recently replied to another similar topic and this is what I found then https://wordpress.stackexchange.com/questions/99849/search-that-will-look-in-custom-field-post-title-and-post-content
