Why doesn't nginx update the fastcgi cache in this case?

Question

PHP

fastcgi_cache_path /var/tmp/cache-html levels=1:2 keys_zone=cache_html:16m inactive=1y max_size=4g;

location /index.php {
    internal;
    try_files /index.php =403;
    include fastcgi_params;
    fastcgi_pass unix:/etc/alternatives/php-fpm.sock;
    set_by_lua_block $key {
        return ngx.var.request_language .. '/' .. ngx.var.request_uri:sub(1, (ngx.var.request_uri:find('?') or (ngx.var.request_uri:len() + 1)) - 1):gsub('[/\\]+', '/')
    }
    fastcgi_cache_key $key;
    fastcgi_cache cache_html;
    fastcgi_cache_min_uses 1;
    fastcgi_cache_lock on;
    fastcgi_cache_background_update on;
    fastcgi_cache_use_stale updating;
    gzip off;
    gunzip on;
}

php (fastcgi) sends the X-Accel-Expires (+ Expires) header in the response, and in accordance with it, nginx caches this response.
And it seems like the norms work, but occasionally it happens that the cache is not updated (that is, the old page is constantly displayed).
I found this page in the cache, I look - and indeed:
X-Accel-Expires: @1598649662
I.e. overdue yesterday.

In general, judging by the description of fastcgi_cache_background_update: on

Allows you to run a background subrequest to update an expired cache item while a stale cached response is being returned to the client. Using an obsolete cached response at the time it is being updated should be allowed

(allowed with fastcgi_cache_use_stale_updating: updating), it seems that the page simply has the status "updating" for some reason, but in fact there is no update.

So what happens? And what to do with it?