I have the harness use the same Armature as the avatar in blender (and skinned to the avatar mesh, or course), then export it as either FBX or glTF to Resonite, where it's then made into a playable avatar in-game. I don't play on VRC, so I don't now how anything is set up there.
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.
Been doing this 3D-modeling thing for almost 20 years and I've never come across anything like this. Why would everything work perfectly fine up until the point it's made a playable asset makes no sense. Like from where does the harness find a different armature to follow? Does the plugins you have add one?
Though, I'll mention again the previously mentioned animation Action sets the Viwi armature has in it. They have names like "Action.001", "Export Pose", "FluffyTailPose". They were actually a major headache for me when I was making the Viwi into an avatar on Resonite.
In fact, when I play a few keys forward on any of the Action poses, the armature plays into the Rest Pose. And from the look of things, that thigh strap clipping in like that for you is where it would be when in rest pose.
So, if there is a way to remove those Action poses from the armature, we might be on to something?