Yea, I thought with the zones, if the original mesh was welded, the effect of the joint would be "in harmony" ;-) an everything affected the same way.

In the meanwhile, I checked the morphs the P8 morph brush creates: And they also seem to maintain identical values at the welded verts.

The funny thing is, I often have situations where letting my program move each vertex on its own, welded or not, makes morphs that are not "in harmony" at welding points but produce a much better shading. However, in other situations, the contrary is true.

Also, I have trouble telling the poser support what my problem is.
I assume, the "default" way is to have a morph that has identical values for welded verts, and the shading errors that seem to occur in extreme poses are, well, bad luck. But they seem to not want to tell me a clear statement.