Character's lower legs levitate every time they bend their knees?

[Plu1223]

I've been recently trying to port a model in for the Character Select mod while following the tutorial that Garrulous64 made recently, but when grounded animations are being preformed, their lower legs and their feet start to levitate like the picture I provided. (They're doing the idle/standing animation btw)

Would anybody know how to fix this? Thanks!