Prince Yi was having a good day, not at all bothered by the fact that Imperial guards were "guarding" his residence. Although he did not expect that turn in his conversation with the Emperor in the main hall, he reasoned that it was inconsequential. After all, that conversation had nothing to do with the matter at hand, which was a foreign pregnant Princess.
As Prince Yi had expected, the Emperor placed him on house arrest but he was not bothered at all. After all, it was not like Prince Yi had anywhere else he actually wanted to be. Princess Nalan was stuck in the palace so it was not like he could go see her, not that he actually felt inclined to at this point. Moreover, she was already pregnant which even made this house arrest seem more pointless since there was nothing this was preventing. Prince Yi surmised this whole house arrest episode to simply be the Emperor showing off his power to soothe his bruised ego.