is_singular() in mu-plugins not working

When the active_plugins option is retrieved, WordPress does not yet know what page has been requested, so this approach will not work.

In order to achieve your objective, you would selectively activate the plugin (rather than selectively deactivate the plugin), but this may cause problems and destabilize your site (untested):

add_filter( 'option_active_plugins', static function ( $plugins ) {
    if ( is_admin() ) {
        return $plugins;
    }

    $key = array_search( 'quiz-maker/quiz-maker.php', $plugins );

    if ( false === $key ) {
        return $plugins;
    }

    unset( $plugins[ $key ] );

    return $plugins;
} );

add_action( 'template_redirect', static function () {
    if ( ! is_singular( 'post' ) ) {
        return;
    }

    // Code to load the plugin.
} );

It’s very likely this will not work. In which case, you’ll need to reach out to the plugin author with your request, which, in my experience, they’re likely to ignore.