Nginx代理缓存过期到期

我使用nginx 1.9.9作为Gravatar的代理(将化身的等待时间从300ms减少到15ms).但是,当我指定缓存在7天后过期时,缓存将在几分钟后到期.在短短几分钟内,我收到标题X-Proxy-Cache：EXPIRED.

配置如下：

proxy_cache_path /var/www/gravatar-cache levels=1:2 keys_zone=gravatar:8m inactive=7d use_temp_path=off max_size=1000m;
proxy_cache_key "$scheme$request_method$host$request_uri";

server {
    listen 443 ssl http2;
    server_name [hidden];

    access_log off;

    ssl_certificate [hidden];
    ssl_certificate_key [hidden];

    location /avatar/ {
        expires 365d;
        proxy_cache gravatar;
        proxy_cache_valid any 7d;
        proxy_pass https://secure.gravatar.com;
        proxy_ignore_headers Cache-Control;
        proxy_hide_header link;
        proxy_hide_header source-age;
        proxy_hide_header content-disposition;
        proxy_hide_header via;
        proxy_hide_header x-varnish;
        proxy_hide_header accept-ranges;
        proxy_hide_header last-modified;
        add_header Cache-Control public;
        add_header X-Proxy-Cache $upstream_cache_status;
    }
}

配置有问题吗？

谢谢！