Seems like an issue with acf_form, I’ve tried using a plugin Advanced forms to do something similar, and it submits no issues. I’d rather stick to the original acf-form if possible.
This is where the warnings are pointing to in pluggable.php
/**
* Filters the X-Redirect-By header.
*
* Allows applications to identify themselves when they're doing a redirect.
*
* @since 5.1.0
*
* @param string $x_redirect_by The application doing the redirect.
* @param int $status Status code to use.
* @param string $location The path to redirect to.
*/
$x_redirect_by = apply_filters( 'x_redirect_by', $x_redirect_by, $status, $location );
if ( is_string( $x_redirect_by ) ) {
header( "X-Redirect-By: $x_redirect_by" );
}
header( "Location: $location", true, $status );
return true;
}
endif;
