Marketo Unique Code is a system managed field. Any particular reason why you would want to use this field and also display it to the lead?
Also, how do you validate that there is an actual value for this field on the lead record? It's not in the lead info view or can't be added to smart list views as a column from what I can tell.
There's never been a guarantee that the form data will be fully reconciled with the lead database by the very next pageview. The web session is guaranteed to be eventually consistent, but AFAIK not more than that.
I think some users believe they've "gamed" this rule, but it's still in place as far as I can see across the instances I work on.