thats just nonsense fear. I did hundreds and hundreds of trades yet villagers never showed customs designs NOT displayed at tailor. So it means that some visitor sneaked inside the tailor and put up their design. the host didn't notice and villager wore it. its impossible for a villager to wear something not displayed. I've been playing since launch and put up designs for villagers in tailor. So I would have noticed a foreign design immediately.
We don't need this custom design thing transform into another animal crossing urban legend please lol
I find the fact that visitors can upload custom designs in our tailors' shop really surprising, given how keen Nintendo usually are to protect players online - particularly children.
You'd think they'd make it so that only your best friends can upload designs, at least.