Reverse Proxy apparently not working location rules

I have the following nginx settings:

upstream cluster_web {
ip_hash;
server 192.168.115.82 weight=3;
server 192.168.115.92 weight=2;
server 192.168.115.100 weight=2;
server 192.168.115.95 weight=3;
server 192.168.115.103 weight=1;
}

server {
location /app1 {
proxy_pass http://192.168.115.92;
}

location / {
proxy_pass http://cluster_web;
}
}

Users accessing /app1/ after accessign another pages (from the / location) aren't always being redirected to the specified server on location /app1. What am I missing?

Best regards