Autocomplete Rule on "Assignment Category"

Hi everyone!

We have configured an Autocomplete Rule on "Assignment Category", if assignment category is "Operational staff not subject to dock" then the assignment category field must not be filled in and must return me an error. We created the rule, but it doesn’t work as we would like: in "Assignment category" when I enter the value I don’t get an error. The error occurs when I try to change the value a second time.

How can we solve it?

Regards,

Mario