I think the exterior glitch occurs when the person TTs backwards when a new villager is due to move in. For example, if Marshal moved out, then Audie took his place, if the person TT-ed backwards, the house would have Marshal's exterior but Audie's interior. I'm not 100% sure on this, but adopting a viallger with a glitched exterior should be ok as I've seen people say their exterior retruns to normal.