After testing a little more in depth, it looks like problem is worst.
Vertex group feature of modifier is crashing blender when any modifier is used before Weld modifier.

I suppose it is due to lack of support of custom data. But I don't face just a wrong interpolation but an immediate crash.

Like I said in my previous comment, same problem is happening with other modifiers.

Now, in master 77d82ed, there is no more problem with vertex weight modifiers.

But, following same principle : Weld modifier using a vertex group preceded by another modifier ; it is still producing a crash for many modifiers.

I obtained a crash with :
for Modify category : Normal Edit, Weighted Normal
for Generate category : Array, Bevel, Build, Decimate, Edge Split, Mirror, Multiresolution, Remesh, Screw, Solidify, Subdivision Surface, Triangulate, Wireframe
for Simulate category : Explode, Fluid Simulation, Ocean, Particle Instance, Smoke

There is no crash with deform modifiers.