I'm not seeing the slipping so much in your video. It looks a bit jerky however. But maybe this is just walk designer? I've never used it. I've only ever done simple animations, never any walks. I did see another Supergirl walking animation though, and this one is similarly jerky. Looks like Poser.

https://m.youtube.com/watch?v=V5bavdlVHjo

I'm seeing that super busty Supergirl is a thing so mine is henceforth getting a breast reduction lol. Like that yours has a skirt though.