Unsure if characterizing this as a bug is accurate. I know on Judge Center I could find the judge I was attempting to create an exam for by using their DCI #, to ensure I got that judge and not someone who might have the same name. When I attempt to create an exam on JudgeApps, inputting the judge’s DCI number outputs an error message.
It’s an auto-complete field, like any other field on Apps where you need to select a specific judge: start typing in their name, and click on the right judge from the list of names that appears in the dropdown menu.