VTK is not supporting collapsed (degenerated) nonlinear cells
I have an example of a single tri-quadratic hexahedron where all the points of one face ( 9 points ) are merged into a single point. So it looks like it is a tri-quadratic pyramid, but the cell type is a tri-quadratic hexahedron.
Degerated_tri-quadratic-hex.vtu
When you increase the nonlinear subdivision level, here is what you get:
The feature edges bug might be linked to this issue that occur with a degenerate quads paraview/paraview#21614 (closed)
Edited by Florian Maurin