Even with a vanilla installation, I can’t get it to work consistently. It may be my hosting or the caching system, but I’ve decided to go with a javascript reload to cover my bases.

add_filter( 'fep_form_submit_button', function( $button, $where ){
	if( 'reply' == $where ){
		// $fep_message_id = fep_get_the_id();
		// $website = site_url( '/messages/', 'https' ) . '?fepaction=viewmessage&fep_id=' . fep_get_parent_id($fep_message_id);
		$button = '<button type="submit" onclick="setTimeout(location.reload.bind(location), 1500);" class="fep-button" name="fep_action" value="'. esc_attr( $where ) .'">Reply</button>';
	}
	return $button;
}, 10, 2 );