Import the 1.3 obj included directly into Poser. Unwelded. Import V4 obj or La Femme obj directly into Poser. Welded.

The same result with this above will be same in the other softwares.

If you're are saying the obj is not unwelded, my point, then there's something seriously wrong with ALL of my importers in all of my softwares. Not just Blender, which behaves normally with the welded 1.2 object. But with Mudbox, Zbrush, Poser, and Blender.

And I know what I'm doing with stacked UVs and UDIM. Stacked sucks because you cant paint across seams as easily. If there's no advantage why are all softwares adopting it? Higher resolution textures with less overhead and cross UV shell painting would probably be the reasons.

Sigh. Just ignore my observations. Obviously the included mesh is welded ;)