Having the same problem. In my case it's an avatar with a skirt, which is rigged to both the HIps root bone as well as each thigh bone. Aside from a few fringes when looking from the side (which has high weighting from both sources combined), all other surfaces are wiped out. Interestingly some of the underlying thighs, also connected to the thigh bones, were intact up past the point where the skirt was lost. There was a transform on the legs, but removing it had no effect. There's a gap between the thigh bones and Hips root, rather than any intermediate bones being contiguous with it. That means there's also a fair distance between the tail of the Hips root, pointing up, and the head of the thigh bones. This seems to go against "best practice" but it hasn't had any serious repercussions in-game. Wonder if it plays a role?