NPC Sims that attend Festivals are not longer seen in not festival appropriate outfits.
Fixed in Patch 1.58.63
I'm going to stick a "Me too" and confirm this one.
Back in February when they fixed the strangely dressed NPCs at festivals, they fixed all festivals except for GeekCon and I commented there, but the thread is marked as "feedback" and so it was probably overlooked.
The some of the situational outfits for visitors to GeekCon seem bugged, so they appear in random clothing or without pants on.
I also notice that I no longer see sims walking around in Pro Gamer outfits (jacket and cap like the sims wear if they are in the eSports branch of Tech Guru) which I remember seeing in older videos of let's plays of City Living.
And the cosplayers seem to only wear a few of the available outfits now - I haven't seen any sims wearing the Star Wars outfits for several patches now. No Princess Leias, No Vaders, No Twii'Lek women.
Edit: Spotted a Princess Leia, a Darth Maul and a pro gamer outfit on a townie doing the UGT, so those still work, just rarer than I expected.
Perhaps some tagging errors in the clothing and the game is trying to use those, but failing?
If they do fix this, can they also tag a few more of the costumes they added to the game for cosplayers at GeekCon, like the Final Fantasy XV Noctis costume? I'd love to see sims walking around in that.
