I'm using a relationship form in a modal window...not sure if that matters.
Even though I have it set to Ajax (and even when I don't) and "stay on same page" after submit it always redirects to the posts page.
You can change the form's redirect to staying on the same page, or redirect back where you (or the user) came from, or redirect to a specific page/post
If you do not specify a post to redirect to, it has no post to redirect to and hence it is unlikely that after submitting a Relationship form it would redirect to any post, if not specified in the form.
Locally when I test, it works just fine (with AJAX and remaining on the page).
Maybe you need to re-save that form?