为什么 NGINX 使用自定义变量代理 ws 会异常？

之前一直用同样的方式连接后端服务，是正常使用的。如

location ^~/prod-api/ {
	    proxy_pass http://$api_host:$api_port;
	    ......
	}

异常情况:

连接 http://$api_host:8877/ws/ 就会异常

server {
        listen    443 ssl;
        set $api_host api;
        ..........

location  /ws/ {
            proxy_pass http://$api_host:8877/ws/;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "Upgrade";
            proxy_connect_timeout 90s;
            proxy_send_timeout 120s;
            proxy_read_timeout 120s;
            send_timeout 120s;
        }
}

正常情况:

设置 http://api:8877/ws/ 正常

server {
        listen    443 ssl;
        set $api_host api;
        ..........

location  /ws/ {
            proxy_pass http://api:8877/ws/;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection "Upgrade";
            proxy_connect_timeout 90s;
            proxy_send_timeout 120s;
            proxy_read_timeout 120s;
            send_timeout 120s;
        }
}

在 nginx 容器内两种设置的抓包截图:
https://imgur.com/a/XzyNFQ6

搞不清楚是什么原因导致这种情况，为什么不能这样配。想请教下大家