Nginx outputs incorrect website content when configuring reverse proxy

when I add proxy_set_header Host $host; , there is a multi-site configuration on the host pointed to by the reverse proxy. I expect to display the content of site A, but actually show the content of site B. I don"t know what the problem is. The following is my reverse proxy configuration.

user  www www;
worker_processes auto;
error_log  /home/wwwlogs/nginx_error.log  crit;
pid        /usr/local/nginx/logs/nginx.pid;
google_perftools_profiles /tmp/tcmalloc;
worker_rlimit_nofile 51200;

events
    {
        use epoll;
        worker_connections 51200;
        multi_accept on;
    }
http
    {
        include       mime.types;
        default_type  application/octet-stream;

        server_names_hash_bucket_size 128;
        client_header_buffer_size 32k;
        large_client_header_buffers 4 32k;
        client_max_body_size 50m;

        sendfile   on;
        tcp_nopush on;

        keepalive_timeout 60;

        tcp_nodelay on;

        fastcgi_connect_timeout 300;
        fastcgi_send_timeout 300;
        fastcgi_read_timeout 300;
        fastcgi_buffer_size 64k;
        fastcgi_buffers 4 64k;
        fastcgi_busy_buffers_size 128k;
        fastcgi_temp_file_write_size 256k;

        gzip on;
        gzip_min_length  1k;
        gzip_buffers     4 16k;
        gzip_http_version 1.1;
        gzip_comp_level 2;
        gzip_types     text/plain application/javascript application/x-javascript text/javascript text/css application/xml application/xml+rss;
        gzip_vary on;
        gzip_proxied   expired no-cache no-store private auth;
        gzip_disable   "MSIE [1-6]\.";

        server_tokens off;
        access_log off;
        
include vhost/*.conf;

server
{
    listen 80;
    include /usr/local/nginx/conf/domainsA.txt;
    location /
    {
        proxy_set_header Host $host;-sharp 
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $remote_addr;
        proxy_pass http://examplea.com/;
    }
    access_log  /home/wwwlogs/www_accessa.log;
}

server
{
    listen 80;
    include /usr/local/nginx/conf/domainsB.txt;
    location /
    {
        proxy_set_header Host $host;-sharp 
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $remote_addr;
        proxy_pass http://exampleb.com/;
    }
    access_log  /home/wwwlogs/www_accessb.log;
}

/ usr/local/nginx/conf/domainsa.txt is as follows:

server_name example_a.com;

/ usr/local/nginx/conf/domainsb.txt is as follows:

server_name example_b.com;

result of running curl-I http://example_a.com:

HTTP/1.1 200 OK
Server: nginx
Date: Sun, 02 Dec 2018 03:19:24 GMT
Content-Type: text/html; charset=utf-8
Connection: keep-alive
Vary: Accept-Encoding
Vary: Accept-Encoding
X-Powered-By: PHP/7.2.6
Set-Cookie: PHPSESSID=qurm52c7jltd0quhvqud58nd2d; path=/
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-store, no-cache, must-revalidate
Pragma: no-cache

there is no error log in the log directory.
I"ve been looking for a solution for a few days, hoping someone can help me.
my goal is to get the real domain name before the reverse proxy .
related questions

solution: I tried to modify the nginx configuration several times and found a configuration that was useful to me. I still don"t know the cause of the problem. Here is my configuration:

-sharp Host
proxy_set_header X-Real-HOST $host;

// php$_SERVER["HTTP_X_REAL_HOST"]host