If statement with $limit_req_status under location block with proxy_pass not working

[acidiclight]

Hello,

I'm trying to customize my response to rate-limited requests by keeping
limit_req_dry_run on, and using an if statement depending on the value of
$limit_req_status:

This works as expected:

limit_req_zone $binary_remote_addr zone=one:1m rate=2r/m;

server {
    
    listen       80;
    server_name  localhost;

    location / {
        # set rate limiting for this location
        limit_req zone=one burst=10 nodelay;
        limit_req_dry_run on;

        add_header X-my-var "$myvar" always;

        if ($limit_req_status = "REJECTED_DRY_RUN") {
            add_header X-custom-header "rejected" always;
            return 400 'rejected';
        }

        root   /usr/share/nginx/html;
        index index.html;
    }

}

But once I replace root and index with a proxy_pass, the whole thing stops
working:

limit_req_zone $binary_remote_addr zone=one:1m rate=2r/m;

server {
    resolver 8.8.8.8;
    listen       80;
    server_name  localhost;

    location / {
        set $myupstream "myurl.com";
        
        # set rate limiting for this location
        limit_req zone=one burst=10 nodelay;
        limit_req_dry_run on;

        add_header X-limit-req-status "$limit_req_status" always;

        if ($limit_req_status = "REJECTED_DRY_RUN") {
            add_header X-custom-header "rejected" always;
            return 400 'rejected';
        }

        proxy_pass http://$myupstream;
    }

}

I added $limit_req_status to my log_format and can confirm that the value of
$limit_req_status does get set to "REJECTED_DRY_RUN". I also see the header
"X-limit-req-status" from the request set to "REJECTED_DRY_RUN".

I'm assuming the issue is the way Nginx evaluates if statements that have
unset variables at the beginning of the request? If so, any pointers on how
to get this working? Thank you!

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,294407,294407#msg-294407