it floats because its either hitting a collider (that's too big or shouldn't be there) or you have the collision radius or collision power for the clothing too high. but you said its happening with all skirts, so its definitely the related to first. try this, add
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plugin to the person atom, then click show previews, it will show all the colliders, now see if the glute colliders are sticking out way too much or in general any oddities with the colliders.
also check if there is any invisible CUAs or atoms under the skirt. some creators like to use them to reduce clipping