Alias with variable sometimes works and sometimes returns 404

I have the following in my server block:

set $filename wizard-min;
if($cookie_B) {
   set $filename $cookie_B;

location ~* "^/b/([a-f0-9]{10})$" {
   default_type application/javascript;
   alias /var/www/b/b-$filename.js;

   sub_filter_types application/javascript;
   sub_filter '%replace%' '$1';

I request it using curl:

curl -v ‘http://server/b/0123456789

sometimes it returns the file correctly, and sometimes it returns a 404.
When it does return a 404, the error log says:
“/var/www/b/0123456789” failed (2: No such file or directory)

Note that I’m testing this on a single server, and every request goes
the access log file, but some of them (20-30%) return a 404.

If I take out the variable from the alias line, the file returns
every time.

Any idea?

PS: Is there any way to get my code formatted correctly on this forum?

Posted at Nginx Forum: