Ideas
I have townies running around in pirate costumes after pirate day ended, townies wearing festival clothes (city living) after the festivals have ended...
My own sims also never change out of the spa bathrobes unless directed to. I travel home with them from the spa lot, and they arrive in the spa bathrobes, and continue wearing them until I tell them to change. Sometimes when I have them take a shower, they will change back into the robes after, too. Recently I travelled to the library from the spa instead of home, and they arrived in bathrobes in front of the library during a blizzard. It was a bit bizarre. (Is this the same bug, or should it be reported elsewhere? Is this thread only for NPCs or for all situation outfits getting stuck?)
Never seens NPCs in spa robes, though, they wear other random things.