Editing a mesh (inserting new vertices) results in wrong relative shapekeys. I would expect that the location of the relative keys is identical to the basis-key for this vertices. But it isn't. Instead they are somehow computed by somehting like this: "basis.co + (relative.co - (0,0,0))".

Depending on the representation of relative keys, this should be "basis.co" or "(0,0,0)", for every newly inserted vertex.

I appended a simple example with the unexpected result. To reproduce it, just add a loopcut (Ctrl + R) to the middle mesh and step back into object mode. The result will be a more or less Saturn like shape.