Unstable rendering of Nginx images. Where to dig?

Question

Nginx

There is the following image resizing system.
If the image has not yet been generated, then the following link is generated:
/media/myfilter/filename.jpeg?id=6
Here, the original image is retrieved from the database with id = 6, processed by the myfilter filter and simultaneously sent to the user with the mimetype image/ jpeg, and is also saved along the route /media/myfilter/filename.jpeg
Further, when requesting a picture with ID = 6, the database knows that a file with this filter already exists and a link will be generated:
/media/myfilter/filename.jpeg
Where is the web the server will already give the file directly.
The problem is that such files /media/myfilter/filename.jpeg are given unstable, about 80% work, and the rest simply do not come from the server...
OS Debian 8, nginx 1.11.13
tried on 2 vps with 512mb ram and 6gb ram
nginx config:

location @rewriteapp {
  rewrite ^(.*)$ /app.php/$1 last;
}

location ~ ^/app\.php(/|$) {
  fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
  fastcgi_split_path_info ^(.+\.php)(/.*)$;
  include fastcgi_params;
  fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
  fastcgi_param HTTPS off;
  internal;
}

location ~ ^/media {
  access_log off;
  expires 30d;
  try_files $uri @rewriteapp;
}

Has anyone experienced something similar?