When topdecking a Border Guard with Horn, there is no way to choose which Border Guard to topdeck. This usually doesn't matter, except with Prince. It seems that the system automatically chooses to topdeck the first Border Guard played, which is of course the Princed one and exactly the one you probably didn't want to topdeck.
Building an interface to properly resolve this and related bugs sounds hard. But I think there should be an easy effective fix here: just default to topdecking the last Border Guard instead.
Thanks for the report!
You are right that this constitutes a problem for any cards that may be topdeckable during discard. Solving it is unfortunately not that easy because the decision which instance of a card will be topdecked is not linked to "played first" or "played last", it's due to an internal number associated to the card instance.
You could check which instances are in play and sort them according to their spot in the play area.
The way the cards are organized on the client isn't known to the server. I don't know if the server even 'remembers' the order cards where played.
When you Prince a Border Guard, store that card's internal card instance number somewhere, and then when you topdeck a Border Guard, try all of the ones not in that list first before topdecking one that has been Princed.
EDIT: Alternatively you could use the interface you have for Scheming cards instead of for stuff like Alchemists if this fix ends up being easier.
It's clear that something can be done to automate this and not use the Princed version of Border Guard/Scheme/Walled Village (if the player so desires).
I was just clarifying that what appears to be the 'obvious' solution from the outside may not actually be possible or easy.
Adam's idea is in the general sense the way to go, IMO: Make the server aware that certain cards are special for certain effects, which allows the server to later take this into account if automated, and allows this info to be sent to the Client, so you can make the informed decision yourself (topdeck the Band of Misfits that's currently disguising as as Border Guard, or make sure to topdeck the non-Princed Border Guards, etc.).