Wordpress - custom page template will not assign to page - php

Question

First time building a Wordpress plugin. I create a page and I want to assign it a page template from my plugin folder. My code shows the Template name in the selectable list in the page editor, but it is not attached to the page. Nor manually attaching does anything to the page. But the path for the file is correct.
My code to create a page:
$page_path = "this-is-a-campaign-landing-page";
$page_title = 'This is a Campaigns Page Title';
$page_content = 'THIS IS A CAMPAIGNS PAGE BODY';
$page_check = get_page_by_title( $page_path );
$page = array(
'post_type' => 'page',
'post_title' => $page_title,
'post_content' => $page_content,
'post_status' => 'publish',
'post_author' => $author->ID,
'post_slug' => $page_path
);
if (!isset($page_check->ID) && !get_page_by_path($page_path)) {
$page_id = wp_insert_post($page);
if ($page_id) {
$template = '/home/vagrant/src/wptest/wp-content/plugins/pm/campaign_page.php';
update_post_meta($page_id, '_wp_page_template', $template );
}
}
Separately I add the template with add_filters
function add_campaign_template ($templates) {
$templates['campaign_page.php'] = 'Campaign Page';
return $templates;
}
add_filter ('theme_page_templates', 'add_campaign_template');
function set_campaign_template ($template) {
if ('campaign_page.php' == basename ($template)) {
$template = '/home/vagrant/src/wptest/wp-content/plugins/pm/campaign_page.php';
}
return $template;
}
add_filter ('page_template', 'set_campaign_template');
The page creates with no errors. When I view or edit the page, Default template is selected and my template appears in the list. Selecting it manually has no effect. What did I miss?
My simple page template:
<?php
/**
* Template Name: Campaign Page
*
* #package PM
*/
// get_header();
?>
<div id="primary" class="content-area">
<main id="main" class="site-main">
<section class="outer-categories">
<div class="container-fluid">
<div class="row text-justify">
THIS IS THE RIGHT PAGE TEMPLATE
THIS IS THE RIGHT PAGE TEMPLATE
THIS IS THE RIGHT PAGE TEMPLATE
THIS IS THE RIGHT PAGE TEMPLATE
<div class="col-lg-12">
<?php
while ( have_posts() ) :
the_post();
get_template_part( 'template-parts/content', 'page' );
endwhile; // End of the loop.
?>
</div>
</div>
</div>
</section>
</main><!-- #main -->
</div><!-- #primary -->
<?php
// get_footer();
Why is it not adding the template? With debug on I get no error in debug.log

Answer 1

Wow, long day.
Change this:
function set_campaign_template ($template) {
if ('campaign_page.php' == basename ($template)) {
$template = '/home/vagrant/src/wptest/wp-content/plugins/pm/campaign_page.php';
}
return $template;
}
add_filter ('page_template', 'set_campaign_template');
to this:
function set_campaign_template ($template) {
if ('campaign_page.php' == basename ($template)) {
$template = 'campaign_page.php'; <---- FIX
}
return $template;
}
add_filter ('page_template', 'set_campaign_template');