Client can connect to reverse proxy running Debian 10 (Apache/2.4.38) but not Debian 11 (Apache/2.4.54)

I have a website hosted behind 2 reverse proxies running Apache on Debian. The client is an automated script and it's not easy to know what config/software they're using.

When both reverse proxies were running Debian 10 (Apache/2.4.38 (Debian) OpenSSL/1.1.1n) the client was able to connect fine. When I upgraded one proxy to Debian 11 (Apache/2.4.54 (Debian) OpenSSL/1.1.1n), the client started having random TCP timeouts when trying to query our website. I can connect fine using the new proxy with Firefox or Chrome.

After much head scratching, I disabled the Debian 11 proxy on the load balancer and the client timeout problems disappeared.

The apache config is shared between both proxies and is identical. What protocols/cyphers have been disabled between Debian 10 and 11 that my client must be using?

Here's the config, the client queries the /api/ path that triggers mod_rewrite. Sensitive information has been replaced by the word sanitized:

SSLProxyEngine on
SSLProxyVerify none
SSLProxyCACertificateFile /opt/sanitized.cer 
SSLProxyCheckPeerName off 

Listen 10.10.3.52:443
<VirtualHost 10.10.3.52:443>
    ServerName https://sanitized:443
    DocumentRoot /var/www
    
    SSLEngine On

    SSLCertificateFile       certs/live/sanitized/fullchain.pem
    SSLCertificateKeyFile    certs/live/sanitized/privkey.pem

    SSLProtocol             all -SSLv3 -TLSv1 -TLSv1.1
    SSLCipherSuite          ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256
    SSLHonorCipherOrder     on
    SSLCompression          off
    SSLSessionTickets       off

    Header add Strict-Transport-Security "max-age=63072000"

    SSLUseStapling On
    SSLStaplingReturnResponderErrors off
    SSLStaplingFakeTryLater off

    RequestHeader append SSL Enabled

    Header always append X-Frame-Options SAMEORIGIN
    Header add Referrer-Policy "strict-origin-when-cross-origin"

    RewriteEngine On
    RewriteRule ^/api/sanitized/(.+)        https://10.10.4.126/api/sanitized=$1

    ProxyRequests Off
    ProxyPreserveHost On
    ProxyPass / balancer://sanitized/
    ProxyPassReverse / balancer://sanitized/
</VirtualHost>