It's something very simple, I'll explain it in detail but directly.

Foxy in fnaf 1 plus, the original fixed version, which will even be in the DLC, she wears the same outfit, but after being broken, and becomes the version we know as the main one in the first game, which makes sense, since he is declining, but then in the second game when he is worse and more broken he appears with his clothes back on, This happens precisely because the animatronics were going to be restored, so they put Foxy's original clothes back on him.

Fun fact

In the original fnaf plus foxy actually had an outfit, it was a jacket, but after being broken he loses it