Ok, this one is going to be a little weird, but it seems the armbinders too have a very odd problem. When NPCs wear an armbinder their arms are not together enough, resulting in clipping through it. However, the player character does not have this problem even if she is the same body type and sliders as the NPC. To make things even more weird, during bound sex animation, the NPC places her arms properly inside her armbinder. It seems not the size of the binder is bugged, but the pose of the NPC