Tried it now, has fixed the problem. Thanks Steve.

My post type:

 $post_types['resource'] = [
            'labels' => piklist('post_type_labels', 'Resource'),
            'public' => true,
            'description' => 'Resources for download.',
            'supports' => array( 'title', 'thumbnail' ),
            'status' => array(
                'hidden' => array( 'label' => 'Hidden' ),
                'publish' => array( 'label' => 'Published' ),
            ),
            'taxonomies' => array( 'category', 'post_tag' ),
            'publicly_queryable' => false,
            'show_in_menu' => true,
            'query_var' => true,
            'capability_type' => 'post',
            'has_archive' => true,
            'rewrite' => [ 'slug' => 'resources', 'with_front' => false ],
            'hierarchical' => false,
            'show_ui' => true,
            'menu_icon' => 'dashicons-media-document',
            'menu_position' => 21,
            'hide_meta_box' => [
                'submitdiv',
                'author',
                'wpseo_meta',
                'customsidebars',
            ],
        ];

I’ve tried a bunch of things – restarting PHP-fpm, setting permalinks to default, disabling opcache.