Sorry, I deleted the Query LDAP comment because I realized immediately after posting it that you're asking about a form not a workflow... But if it's somehow helpful, great! 🙂
Especially since it works in your testing environment - it seems there would have to be a way to get it to work in production as well. Sometimes I've found with Nintex Forms that if you just delete a control altogether and then create it again from scratch, even if all the settings are identical to what you had before, the new one will work. Might just be a migration hiccup.