custom-page template ignored and single.php used.

I wrote a shortcode that presents the user with a form. I have the form post back to the same page. So

http://somesite/?page_id=36 Takes the user to the page with the shortcode, and the user sees a form.

user fills out form and selects the submit button

form is posted back to http://somesite/?page_id=36.

shortcode code gets exercised again, this time processing the form.

I have an edit for some fields in the shortcode code, and if it fails, an error is added to the form. This all actually works well.

If I pass validation, instead of re-presenting the user with the filled out form page as it should, I get a “query not able to find posts matching…”

What I discovered is that instead of going back to http://somesite/?page_id=36 and using the assigned custom template, single.php is being used. But this is a page, not a post.

I am having trouble tracking down where the error is occurring in template selection, or why. I have gone as far as to comment out most or all the code that get exercised if the form passes validation, but the error still occurs.

Suggestions?

$myId = '191';

	$utility = new PersonUtility();
	if (!isset($_POST['submitPersonForm'])) {
		$id  = isset($_GET['id'])  ? trim($_GET['id'])  : '';
		if(strlen($id) >= 2) {
			$fileName = "wp-content/plugins/rootsPersona/" . get_option("rootsDataDir") . '/' . $id . '.xml';
			if(file_exists($fileName)) {
				$xml_doc = new DomDocument;
				$xml_doc->load($fileName);
				$p = $utility->paramsFromXML($xml_doc);
			}
		}
		$p['action'] =  get_option('siteurl') . '/?page_id=' . $myId;
		return $utility->showForm($p);
	} else {
		$p = $utility->paramsFromHTML($_POST);

		if(strlen($p['id']) < 2) $msg = $msg . "<br />Invalid Id.";
		if(strlen($p['name']) < 1) $msg = $msg . "<br />Name required.";

		if(!isset($msg)) {

			$fileName = "wp-content/plugins/rootsPersona/" . get_option("rootsDataDir") . '/' . $p['id'] . '.xml';
			$xml_doc = new DomDocument;
			if(file_exists($fileName)) {
				$xml_doc->load($fileName);
			} else {
				$xml_doc->load("wp-content/plugins/rootsPersona/" . get_option("rootsDataDir") . '/templatePerson.xml');
			}			

			$xml_doc = $utility->paramsToXML($xml_doc, $p);
			$xml_doc->save($fileName);
			$p = $utility->paramsFromXML($xml_doc);

			$msg = "<br />Saved.";
		}
		$p['action'] =  get_option('siteurl') . '/?page_id=' . $myId;
		return $utility->showForm($p, "<div class='truncate'>" . $msg . "</div>");
		//return $msg;
	}