redirect works on some machines, doesn't on others

Try logging the client IP address, HTTP host header and URI in the rule and see what the client is sending. As someone pointed out recently, the Host header is only required in HTTP 1.1, so some clients might not send it when using HTTP 1.0.

You can use something like this:

when HTTP_REQUEST {
   log local0. "client [IP::client_addr] -> [HTTP::host][HTTP::uri]"
   if { [HTTP::host] contains "old.com" } {
      HTTP::redirect "https://new.com"
   }
}

The log output will be written to /var/log/ltm.

Aaron