多谢大佬 🙏 ，问题已解决，贴上修改后的 nginx 配置供后续遇到问题的筒子们参考：

map $http_upgrade $connection_upgrade {
        default upgrade;
        '' close;
}
server {
        listen 443;
        ssl on;
        ssl_certificate /etc/letsencrypt/live/xxx.cn/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/xxx.cn/privkey.pem;
        server_name note.xxx.cn;
        location / {
                proxy_pass http://127.0.0.1:6806/;
                proxy_set_header Upgrade $http_upgrade;
                proxy_set_header Connection $connection_upgrade;
        }
}