Hi,
It looks like the calculation of a WBS or MindMap node's midpoint ignores the "MinimumWidth" and/or "MaximumWidth" styles when the horizontal alignment is set to the center. This is the case whether the horizontal alignment is declared using "skinparam DefaultTextAlignment center" or a style of "HorizontalAlignment center".
The result is that text is centred, but it does not appear in the middle of the node.
See:
Is it possible to amend PlantUML to support the content centering of nodes with fixed widths?