yes, the other armors do not have the same dangly mesh and do not exhibit this error. the armor also works correct on a human female. (no observable problems on males, too, because again the dangling is much less).
as far as i could see, the difference between the models is only in the vertex positions itself. the dangly part is identical between human and dwarf. also when the animation is frozen (e.g.: when using customize character override), the shirt is aligned propperly. i suppose the null position of the armor is correct and points downward for the dangly part.
maybe the torso is intersecting with parts of the arms and that causes the flex engine to produce a constant sidward force? i do not think a model/animation change secretly slipped into my installation (which might explain why i have the issue on you don't), but it might be a possibility.
i did export the model from the vanilla assets, decompiled it and changed a few things. put it into override. played around with a text editor a bit. therefor i know that it is in fact the vanilla model that is causing this (because i did see it change based on what i did, though nothing seemed to fix the issue)
i'm using a GTX765m with the latest Nvidia driver. (i also can reproduce the issue on a GTX 560m)
i agree, even if this turns out to be GFX/driver related, as long as some work and some don't there is hope to establish something like a "best dangly practice" with respect to more modern (and error prone towards dangly mesh) hardware, if the culprit is found.