Do you know if they performed any negative socials while you weren't watching? That seems to still cause relationship decay even with the mod installed. It doesn't actually have to be a mean action but talking about things others don't want to hear about (such as a hobby) or any conversation they aren't interested in affects relationship totals in the vanilla game. If you notice they will have a minus point over their head etc. sometimes, when they don't like a conversation. Maybe that also affects this mod and still causes decay (it does in reg. game) and why we have the options to change conversations. ETA: Note they say neg. socials will still cause decay.