@Administrator Thank you for your reply. Yes I remember this also with the robber. In the Catan Universe app however if you put a robber on the hex of multiple opponents and select an opponent with no cards you do not get anything from the opponent so this is also a loophole of not robbing the opponent who had cards as it should not give you the option to rob an opponent with no cards and only the ones who have cards if they are on the same hex being robbed. I know that there is the encyclopedia catanica online but it is only in German and I do not trust the translation of my web browser for all these tricky rules.
Secondly the reason I bring up the aqueduct rule is that when playing the baordgame players with aqueduct often forget to pick up their cards if their numbers do not roll whereas the app does not allow the human error to be a factor in the game.