template_include not working in customizer previewer

I already posted a ticket in Trac, because this is maybe a bug rather than a problem. But I haven’t got a reply yet, so maybe someone else knows whats going on or can at least replicate the problem.

If I try to include a different page template via the template_include hook, it doesn’t work within the customizer preview.

I took this example from the Codex:

<?php
function portfolio_page_template( $template ) {

    if ( is_page( 'portfolio' )  ) {
        $new_template = locate_template( array( 'portfolio-page-template.php' ) );
        if ( '' != $new_template ) {
            return $new_template ;
        }
    }

    return $template;
}
add_filter( 'template_include', 'portfolio_page_template', 99 );

For this example my “portfolio-page-template.php” only displays the word “test”:

<?php
echo "test";
?>

If I visit the portfolio page on the frontend it loads the template successfully and displays the word “test”.

But when I try to load the same URL within the customizer via the api it doesn’t loads the template and shows me the home page instead:

wp.customize.previewer.previewUrl.set('https://localhost:8888/portfolio/')

The same happens if I click on a link within the previewer to the portfolio page. It doesn’t load the template nor the page. Any other link or page that doesn’t hook into the template_include loads without a problem.