Request body truncated at ~8k - php

Question

I have two apps, one running in docker deployed in cloud - A - and the other - B - on a dedicated server. Both are written in PHP and using nginx server. App B is doing PUT requests with bigger payloads (let's think about 1M). The problem is that what gets to PHP in app A is truncated to approximately 8k (8209 bytes to be exact). This causes json_decode to fail decoding the request body and the whole request fails.
I have googled and checked configs for quite a long time already but cannot find the issue.
This is my nginx.conf for app A (running in docker in cloud):
worker_processes auto;
user www-data;
pid /var/run/nginx.pid;
error_log /var/log/nginx/error.log warn;
error_log /var/log/nginx.error.log notice;
error_log /var/log/nginx.error.log info;
events {
worker_connections 1024;
multi_accept on;
use epoll;
}
http {
sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 0;
client_header_timeout 60;
client_body_timeout 60;
client_body_buffer_size 10m;
client_max_body_size 100m;
server_tokens off;
reset_timedout_connection on;
send_timeout 60;
include /etc/nginx/mime.types;
default_type text/html;
charset UTF-8;
large_client_header_buffers 4 16k;
fastcgi_buffer_size 128k;
fastcgi_buffers 256 256k;
fastcgi_busy_buffers_size 256k;
fastcgi_temp_file_write_size 256k;
# cache informations about file descriptors, frequently accessed files
# can boost performance, but you need to test those values
open_file_cache max=65000 inactive=20s;
open_file_cache_valid 60s;
open_file_cache_min_uses 2;
open_file_cache_errors on;
log_format timed '$remote_addr - $remote_user [$time_local] "$request" '
'$status $body_bytes_sent $request_time "$http_referer" '
'"$http_user_agent" "$http_x_forwarded_for"';
access_log /var/log/nginx/access.log timed;
upstream backend {
server 127.0.0.1:9000;
}
include conf.d/*;
}
This is my site's conf:
server {
listen 80 default_server;
root /var/www/appB/public;
index index.php;
location = /.well-known/schema-discovery {
add_header Content-Type application/json;
return 200 '{}';
}
location / {
try_files $uri $uri/ /index.php?$query_string;
}
location ~* \.(gif|jpg|png|js)$ {
expires 30d;
}
location ~ \.php$ {
try_files $uri =404;
fastcgi_split_path_info ^(.+\.php)(.*)$;
fastcgi_pass 127.0.0.1:9000;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_param PATH_INFO $fastcgi_path_info;
include /etc/nginx/fastcgi_params;
}
}
This is the www.conf (for php-fpm):
[www]
user = www-data
group = www-data
listen = 127.0.0.1:9000
listen.allowed_clients = 127.0.0.1
pm = dynamic
pm.max_children = 150
pm.start_servers = 30
pm.min_spare_servers = 10
pm.max_spare_servers = 50
pm.process_idle_timeout = 60s
pm.max_requests = 5000
pm.status_path = /status
ping.path = /ping
slowlog = /var/log/fpm/slow.log
request_slowlog_timeout = 60s
request_terminate_timeout = 300s
catch_workers_output = yes
access.log = /var/log/fpm/access.log
php_flag[display_errors] = off
php_flag[html_errors] = off
php_admin_value[error_log] = /var/log/fpm/php_error.log
php_admin_flag[log_errors] = on
php_admin_value[memory_limit] = 1024M
php_admin_value[upload_max_filesize] = 100M
php_admin_value[post_max_size] = 100M
According to the logs neither nginx nor php-fpm do not complain about anything (no errors in the logs).
Does anybody have an idea what might be wrong?
Thanks a lot in advance!