Term count incorrect for custom taxonomy on attachments

[powerbuoy]

When creating a custom taxonomy for the 'attachment' post-type its term counts aren't properly updated.

To reproduce:

1) Download and install WordPress 5.2.4 with the twentynineteen theme

2) Add a custom taxonomy:

<?php
add_action('init', function () {
        register_taxonomy('attachment_category', 'attachment', [
                'labels' => [
                        'name' => 'Attachment categories',
                        'singular_name' => 'Attachment category'
                ],
                'rewrite' => [
                        'with_front' => false,
                        'slug' => 'attachment-categories',
                        'hierarchical' => true
                ],
                'hierarchical' => true,
                'sort' => true,
                'show_in_rest' => true,
                'show_admin_column' => true
        ]);
});

3) Create some taxonomies and assign them to media:

4) Notice how term count still says zero:

[powerbuoy]

Latest WordPress - clean install

[powerbuoy]

Taxonomy created

[powerbuoy]

Assign terms to attachments

[powerbuoy]

Term count still incorrect

[SergeyBiryukov]

Hi there, welcome to WordPress Trac!

Thanks for the report, we're already tracking this issue in #22558.