Feature edges is wrong for all nonlinear cells.
Go to the unstructured grid source. Select for instance a single quadratic hexagon (1x1x1 grid) Switch the representation to feature edge.
We are supposed to get a single cube ( like it is the case for all linear cells). We have instead the following ( note that things get even funnier when you play with the subdivision level).
In the source, replace the quadratic hexagon by other nonlinear cells, the same kind of errors will be produced.
I don't know if this is the same problem, but there was already something already reported for the feature edges: paraview/paraview#21614 (closed)