I built a custom plugin with a form (with form action=admin-post.php and a hidden input for action being the form handler function name) and form handler function, it works as expected on other sites but on this particular site it only works when logged in as admin.

If not logged in, or logged in as a regular user, form submission redirects to site front page. If logged in as admin the form handler function is fired properly.

Any ideas of how to fix?