Enabling http2 on backends in tcp mode

Help!
1

I am trying to configure HAProxy to work with h2 backend using tcp load balancing mode:

listen 443pp
  bind [redacted]:443 interface ens224
  mode tcp
  balance roundrobin
  log global
  source [redacted]
  server backend02 [redacted]:8443 check send-proxy-v2
  server backend03 [redacted]:8443 check send-proxy-v2

The backend is Nginx (serving a static file for testing purpose):

# nginx config:
server {
  listen       *:8443 ssl proxy_protocol ; # works
  listen       *:8443 ssl proxy_protocol http2; # does NOT work

  server_name  backend02-proxy-ssl;

  ssl_certificate           /etc/ssl/private/self-signed.crt;
  ssl_certificate_key       /etc/ssl/private/self-signed.key;

  index  index.html index.htm index.php;
  access_log            /var/log/nginx/ssl-backend02-proxy-ssl.access.log;
  error_log             /var/log/nginx/ssl-backend02-proxy-ssl.error.log;
  
  location / {
    root      /var/www/html;
    index     index.html index.htm index.php;
  }
}

Curl on the load balanced ip fails with this message when http2 is enabled in Nginx:

* Empty reply from server

The Nginx backend is responding to http2 request, tested with curl --haproxy-protocol -k https://[redacted]:8443

HAProxy version:

HAProxy version 2.4.16-1ppa1~focal 2022/04/29 - https://haproxy.org/
Status: long-term supported branch - will stop receiving fixes around Q2 2026.
Known bugs: http://www.haproxy.org/bugs/bugs-2.4.16.html
Running on: Linux 5.4.0-110-generic #124-Ubuntu SMP Thu Apr 14 19:46:19 UTC 2022 x86_64

Nginx version:

nginx version: nginx/1.18.0 (Ubuntu)

OS is Ubuntu 20.04.