Look very closely at the part of the pekes and the policles. Right before the 4 prissy dogs come through when they have to clear a spot, misto/quakso lost his dog foot and 1 out of 2 front dogs coming through kicks it.
Well, it may be a slip-up, but the part of the pekes and the pollicles is supposed to intentionally be full of bloopers, so he may have lost his shoe on purpose, although you'll have to watch how the cats react to this to figure it out.
I asked Jacob Brent (Mistoffelees/Quaxo) if he was supposed to lose his shoe during that scene, and he said no, he wasn't. It may be a wacked out play-within-the-play, but they're supposed to keep all of their costume on. At least he didn't lose his tail...