check if your upstream server has the certificate as well.
check the clients protocols because you provide support only for TLSv1.2
pass some more headers to your upstream
try
ssl_protocols TLSv1.1 TLSv1.2 TLSv1.3;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-Proto $scheme;
try adding support for http2 to eliminate continues handshaking if nginx was compiled with it, like so
listen 443 ssl http2;
it would be helpfull though looking at your upstream logs as well, maybe for a 502
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