game #25000572 I Prince-d a Border Guard and have Horn. I fully understand that if I choose to use Horn to topdeck that BG Prince will lose track of it and fail to set it aside. However, if I also play another BG I should be able to choose to topdeck this BG given Horn's wording "Once per turn, when you discard a BG from play...", right? The client only gives the option to topdeck the first BG played, which appears to me to be a bug. Is this correct?
Yes, that is a bug. The game will choose one Border Guard (or any other card with a similar effect, like Treasury), and that may very well be the one played by Prince that you don't want to topdeck.