Nginx custom 502 page with nested proxies

I know how to create a custom 502 page for an upstream error and did it without problems many times, however in this particular case where one proxied response is passed to another proxy I can't find correct config.

General workflow looks like this: user requests a thumbnail of picture, nginx proxies this request to picture storage. If storage returns 404, then nginx proxies this request to app server that generates a thumbnail with requested URI and returns it with code 200, which is passed to user via nginx.

However, if user requests a thumbnail of completely non-existent picture, the app server returns 502. I'd like to display a custom page in this case instead of built-in nginx "502 Bad Gateway".

As i mentioned in the beginning, I know how to create custom 502 page, but I think this particular complex setup with double proxying requires some extra configuration which I can't find :( I've tried a location with internal option, I've tried a location with direct URL to html page and I've tried "@" localtion, but nginx always displayed its built-in page. Both files 502.html and 502x.html exist in /var/www/html and are readable by nginx:

ll /var/www/html
total 20
drwxr-xr-x 2 root root 4096 May  3 11:21 ./
drwxr-xr-x 4 root root 4096 Mar 24 11:50 ../
-rw-r--r-- 1 root root   36 Apr 28 10:49 502.html
-rw-r--r-- 1 root root   36 May  3 11:21 502x.html

Nginx config:

server {
listen 443 ssl http2;
server_name cdn.domain.tld;

ssl_certificate /etc/letsencrypt/live/cdn.domain.tld/fullchain.pem;
ssl_certificate_key /etc/letsencrypt/live/cdn.domain.tld/privkey.pem;

client_max_body_size 10m;

client_body_buffer_size 128k;
proxy_connect_timeout 75;
proxy_send_timeout 300;
proxy_read_timeout 500;
proxy_buffer_size 4k;
proxy_buffers 4 32k;
proxy_busy_buffers_size 64k;
proxy_temp_file_write_size 64k;
proxy_http_version 1.1;
proxy_buffering on;
proxy_redirect off;

proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Forwarded-Proto $scheme;

#locations for redirection of 502 response received from thumbnail generator. Not working :(
#error_page 502  @502;
#error_page 502  https://cdn.domain.tld/502x.html;
error_page 502  /502.html;
location =/502.html {
   root /var/www/html;
   internal;
}
location @502 {
   root /var/www/html;
   try_files /502x.html 502;
}
location = /502x.html {
   root /var/www/html;
   index 502x.html;
}

location  / {
    proxy_pass http://192.168.10.5/thumbs$request_uri;

    #Sets caching time for different response codes. If only caching time is specified  then only 200, 301, and 302 responses are cached.
    proxy_cache_valid 30m;

    add_header X-Proxy-Thumbs-Cache $upstream_cache_status;

    #intercept 404 from backend
    #from nginx docs:
    #If an error response is processed by a proxied server or a FastCGI/uwsgi/SCGI/gRPC server,.
    #and the server may return different response codes (e.g., 200, 302, 401 or 404), it is possible to respond with the code it returns (equal sign does that)
    #So we proxy 404 to app which generates thumbnail and returns it with 200 code
    proxy_intercept_errors on;
    error_page 404 = @not-found;

    access_log /var/log/nginx/cdn.access.log cachelog;
    error_log /var/log/nginx/cdn.error.log;
}

location @not-found {
   proxy_pass http://192.168.10.12/thumbgen?key=$request_uri;

   add_header X-Proxy-Thumbs-Cache2 $upstream_cache_status;

   proxy_intercept_errors on;

   access_log /var/log/nginx/cdn-404.access.log cachelog;
   error_log /var/log/nginx/cdn-404.error.log;
}

}