[SOLVED] Referrer-Policy and Content-Security-Policy broken on Nginx-only setup

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Rewriting config from scratch, without relying on the previous one, actually did a trick. This one works perfectly for my scenario:

current nginx conf, which passes all tests as of 15.0.7 
upstream php-handler {
    server unix:/var/run/php/php7.3-fpm.sock;
}

server {
    listen 80 ;
    listen [::]:80 ;
    server_name cloud.intra;
    return 301 https://$host$request_uri;
    error_page 404 /var/www/core/templates/404.php;
    return 404;
}

server {
    listen [::]:443 ssl http2 ipv6only=on;
    listen 443 ssl http2;
    server_name cloud.intra;

    index index.php;
    error_log /var/log/nginx/nextcloud-error.log;
    access_log /var/log/nginx/nextcloud-access.log combined;

ssl_certificate /etc/nginx/ssl/cloud.intra/cloud_intra_trust.crt;
ssl_certificate_key /etc/nginx/ssl/cloud.intra/cloud.intra.key;

ssl_session_cache shared:SSL:50m;
ssl_session_timeout 1d;
ssl_session_tickets off;
ssl_prefer_server_ciphers on;
ssl_protocols TLSv1.3 TLSv1.2; # Requires OpenSSL 1.1.1+ - https://serverfault.com/questions/962072/nginx-1-15-10-tlsv1-3-doesnt-get-applied-despite-the-config
ssl_ciphers EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA512:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:ECDH+AESGCM:ECDH+AES256:DH+AESGCM:DH+AES256:RSA+AESGCM:!aNULL:!eNULL:!LOW:!RC4:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS;
ssl_stapling on;
ssl_stapling_verify on;
resolver 8.8.4.4 1.1.1.1 1.0.0.1 8.8.8.8;


error_page 403 /core/templates/403.php;
error_page 404 /core/templates/404.php;
add_header Strict-Transport-Security "max-age=15768000; includeSubDomains; preload;";
# We won't end up in browsers' lists because fuck me that's why, but it satisfies the security check-up

add_header X-Content-Type-Options nosniff;
add_header X-XSS-Protection "1; mode=block";
add_header X-Robots-Tag none;
add_header X-Download-Options noopen;
add_header X-Permitted-Cross-Domain-Policies none;
add_header Referrer-Policy no-referrer;

# Remove X-Powered-By, which is an information leak
fastcgi_hide_header X-Powered-By;

# Path to the root of installation
root /var/www;

location = /robots.txt {
    allow all;
    log_not_found off;
    access_log off;
}

location = /.well-known/carddav {
  return 301 $scheme://$host/remote.php/dav;
}
location = /.well-known/caldav {
  return 301 $scheme://$host/remote.php/dav;
}

# set max upload size
client_max_body_size 900G; # Requires consistent value all across nginx + php-fpm + nextcloud
fastcgi_buffers 64 4K;

# Enable gzip but do not remove ETag headers
gzip on;
gzip_vary on;
gzip_comp_level 4;
gzip_min_length 256;
gzip_proxied expired no-cache no-store private no_last_modified no_etag auth;
gzip_types application/atom+xml application/javascript application/json application/ld+json application/manifest+json application/rss+xml application/vnd.geo+json application/vnd.ms-fontobject application/x-font-ttf application/x-web-app-manifest+json application/xhtml+xml application/xml font/opentype image/bmp image/svg+xml image/x-icon text/cache-manifest text/css text/plain text/vcard text/vnd.rim.location.xloc text/vtt text/x-component text/x-cross-domain-policy;

location / {
    rewrite ^ /index.php$request_uri;
}

location ~ ^\/(?:build|tests|config|lib|3rdparty|templates|data)\/ {
    deny all;
}
location ~ ^\/(?:\.|autotest|occ|issue|indie|db_|console) {
    deny all;
}

location ~ ^\/(?:index|remote|public|cron|core\/ajax\/update|status|ocs\/v[12]|updater\/.+|oc[ms]-provider\/.+)\.php(?:$|\/) {
    fastcgi_split_path_info ^(.+?\.php)(\/.*|)$;
    include fastcgi_params;
    fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    fastcgi_param PATH_INFO $fastcgi_path_info;
    fastcgi_param HTTPS on;

    fastcgi_param modHeadersAvailable true;
    fastcgi_param front_controller_active true;
    fastcgi_pass php-handler;
    fastcgi_intercept_errors on;
    fastcgi_request_buffering off;
}

location ~ ^\/(?:updater|oc[ms]-provider)(?:$|\/) {
    try_files $uri/ =404;
    index index.php;
}

location ~ \.(?:css|js|woff2?|svg|gif)$ {
    try_files $uri /index.php$request_uri;
    # Duplicated, but it works
    add_header Cache-Control "public, max-age=15778463";
    add_header Strict-Transport-Security "max-age=15768000; includeSubDomains; preload;";

    add_header X-Content-Type-Options nosniff;
    add_header X-XSS-Protection "1; mode=block";
    add_header X-Robots-Tag none;
    add_header X-Download-Options noopen;
    add_header X-Permitted-Cross-Domain-Policies none;
    add_header Referrer-Policy no-referrer;

    # Keep nextcloud from thrashing logs queue
    access_log off;
}

location ~ \.(?:png|html|ttf|ico|jpg|jpeg)$ {
    try_files $uri /index.php$request_uri;
    access_log off;
}

}

Marking as solved since all the security requirements from web interface, online-scan at scan.nextcloud.com and SSLLabs’ Server test were met.

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