Here are two copys of the default David geometry exported as a mesh out of Poser.

I choose "Include Body Part Names in Polygon Groups" for one and "Include Existing Groups in Polygon Groups" for the other.

As some smoothing with the MorphBrush quickly reveals, the mesh is split.

But once it's turned back into a figure, Poser welds it all back together.

Welding it during export or import, or inbetween in a modeller, will change the vertex order, and so make all previous morphs unuseable.

If I need to work on an actual welded mesh (Like for making JCMs), I use PoseMorphLoader.

Poser never had a problem with that workflow, so, no, I'm pretty sure the welding/unwelding really isn't part of the problem.