Actually, the 3D visualisation is behaving exactly as we intended.
It was one of our basic design decisions that the visualisation should show what the model looks like as much as possible.
As you might know, in Ple4Win each element only has one diameter, there are no tapered elements. If the user defines a pipeline section with changing diameter, Ple4win approximates this by giving each element in that section an average diameter. See the data of the PIPEDIM output table in the screenshot below. The 3D visualisation shows exactly that, a pipeline with step changes in diameter.
The aim of the 3D visualisation is to give the user a clear view of the model. Its aim is not to give a smoothed, idealized "improved" view of the pipeline. For the same reason all bends are shown as collections of straight elements instead of nicely curved pipes.