Not with the avatar in it.
so basically in unity you have to pair the armature of your clothing to the base avatar armature. There in lies the difference as resonite does that part for you (I assume) but for vr chat you do it yourself either with vrcfury a plugin that does the manual stuff automatically or you can parent it manually by dragging all the bones into the hierarchy. This is where the problem is as there seems to be extra influence where it’s not needed but i cannot find where in the blender file
On the screenshot I showed, I had done what I always have with Resonite. I bring the avatar, with the asset on it and turn that into an avatar in-game, assuming you do the same in Unity for the purpose of VRC. So, you put it all together in blender. Not assign , in this case the harness, later in Reso'/Unity.
And just to test; if you've got it set up and prior to pressing Play, select the armature and pose the thigh if it's still consistent, or it breaks as well at that point?
it works fine before the armatures are linked which must mean that there is some extra influence on the armature when they are linked together but alas i cannot find what that may be im going to start removing some bones that are not needed and see if that makes any difference
EDIT - i removed every bone that has no influence on the weights and its till not working correctly.