access_log with if does not work when variable name starts with a number

[taoky@…]

The nginx.conf for reproducing looks like this:

error_log /dev/stdout info;
pid /tmp/nginx/nginx.pid;

events {
    worker_connections  1024;
}


http {
    include       /etc/nginx/mime.types;
    client_body_temp_path /tmp/nginx/;
    fastcgi_temp_path /tmp/nginx/fastcgi/;
    uwsgi_temp_path /tmp/nginx/uwsgi/;
    scgi_temp_path /tmp/nginx/scgi/;
    server {
        listen 8000 default_server;
        listen [::]:8000 default_server ipv6only=on;
        root /tmp/;
        server_name localhost;
        client_max_body_size 100m;
        set $123_test 0;
        access_log /tmp/access.log combined if=$123_test;
        #set $test_123 0;
        #access_log /tmp/access.log combined if=$test_123;
    }
}

When $123_test is set to 0 in the server block, it will still write to /tmp/access.log as if it does not evaluate to "0". However, if changing its name to $test_123, nothing will be logged by access_log.

It seems that the if in access_log only looks at the first character in this case. When the variable's name is changed to $0123_test, it will also log nothing, no matter its evaluation result.

I have read the documentation of ngx_http_log_module and I think that it looks like a bug, as variables starts with digits work well other places, such as in if block.