WordPress Widget Multiselect Issue

If someone is looking at this question, I managed to find a solution.
1. Switched to a checkbox instead of a multi-select and used “checked” instead of “selected”;
2. Used tax_query and some other query modifications.

The code is not perfect but it works for now.

Working Widget Code for Displaying the last 10 entries in a CPT sorted by single or multiple categories:

class CPT_Widget extends WP_Widget 
{

    public function __construct() 
    {
        parent::__construct(
            'widget_category_posts', 
            _x( 'CPT Widget', 'CPT Widget' ), 
            [ 'description' => __( 'Display the latest entries in your custom post type sorted by single or multiple categories.' ) ] 
        );
        $this->alt_option_name="widget_category_posts";

        add_action( 'save_post', [$this, 'flush_widget_cache'] );
        add_action( 'deleted_post', [$this, 'flush_widget_cache'] );
        add_action( 'switch_theme', [$this, 'flush_widget_cache'] );
    }

    public function widget( $args, $instance ) {
        $cache = [];
        if ( ! $this->is_preview() ) {
            $cache = wp_cache_get( 'widget_cat_posts', 'widget' );
        }

        if ( ! is_array( $cache ) ) {
            $cache = [];
        }
        if ( ! isset( $args['widget_id'] ) ) {
            $args['widget_id'] = $this->id;
        }
        if ( isset( $cache[ $args['widget_id'] ] ) ) {
            echo $cache[ $args['widget_id'] ];
            return;
        }
        ob_start();
        $title          = ( ! empty( $instance['title'] ) ) ? $instance['title'] : __( 'Category Posts' );
        $title          = apply_filters( 'widget_title', $title, $instance, $this->id_base );
        $number         = ( ! empty( $instance['number'] ) ) ? absint( $instance['number'] ) : 5;
        if ( ! $number ) {
            $number = 5;
        }
        $cat_id         = $instance['categories'];
        if( true === $random ) {
            $query_args = [
                'posts_per_page'    => 10,
                'post_type'         => 'yourcustomposttype',
                'orderby'           => 'rand',
                'tax_query' => array(
                            array(
                            'taxonomy' => 'category',
                            'field' => 'term_id',
                            'terms' => $cat_id,
                             ),
                          ),
            ];
        }else{  
            $query_args = [
                'posts_per_page'    => 10,
                'post_type'         => 'yourcustomposttype',
                'orderby'           => 'rand',
                'tax_query' => array(
                            array(
                            'taxonomy' => 'category',
                            'field' => 'term_id',
                            'terms' => $cat_id,
                             ),
                          ),
            ];

        }
        $q = new WP_Query( $query_args );

        if( $q->have_posts() ) {
            echo $args['before_widget'];
            if ( $title ) {
                echo $args['before_title'] . $title . $args['after_title'];
            }               
            while( $q->have_posts() ) {
                $q->the_post(); ?>

                <article id="post-<?php the_ID(); ?>" <?php post_class(); ?>> 

                    <header class="entry-header">
                        <?php the_title( '<h1 class="entry-title"><a href="' . esc_url( get_permalink() ) . '" rel="bookmark">', '</a></h1>' ); ?>
                    </header><!-- .entry-header -->
                        <div class="post-thumbnail">
                            <?php the_post_thumbnail(); ?>
                        </div><!--/.post-thumbnail-->
                        <div class="entry-summary">
                            <?php the_excerpt(); ?>
                        </div><!-- .entry-summary -->
                    <?php } ?>
                </article><!-- #post-## -->
                <?php 
            wp_reset_postdata();
        }
            echo $args['after_widget']; 

        if ( ! $this->is_preview() ) {
            $cache[ $args['widget_id'] ] = ob_get_flush();
            wp_cache_set( 'widget_cat_posts', $cache, 'widget' );
        } else {
            ob_end_flush();
        }
    }


public function update($a, $b) {
  return array(
    'title'      => isset($a['title']) ? strip_tags($a['title']) : $b['title'],
    'categories' => isset($a['categories']) ? array_filter(array_map(function($id) { return intval($id); }, (array) $a['categories'])) : (array) $b['title']
  );
}

    public function flush_widget_cache() 
    {
        wp_cache_delete('widget_cat_posts', 'widget');
    }

    public function form($instance) {
  $title = isset($instance['title']) ? $instance['title'] : '';
  $categories = isset($instance['categories']) ? $instance['categories'] : array();
  ?>

  <p>
    <label for="<?php echo $this->get_field_id('title') ?>">
      <?php _e( 'Title:' ) ?>
    </label>
    <input class="widefat"
           id="<?php echo $this->get_field_id('title') ?>"
           name="<?php echo $this->get_field_name('title') ?>"
           value="<?php echo $title ?>" />
  </p>

  <p>Categories</p>
  <ul>
  <?php foreach (\get_categories() as $category): ?>
    <li>
      <label>
        <input type="checkbox"
             class="checkbox"
             name="<?php echo $this->get_field_name('categories') ?>[]"
             value="<?php echo $category->cat_ID ?>"
             <?php checked(in_array($category->cat_ID, $categories)) ?> />
        <?php echo $category->name ?>
      </label>
    </li>
  <?php endforeach ?>
  </ul>
  <?php
}
}


add_action( 'widgets_init', function () 
{
    register_widget( 'CPT_Widget' );
});