Hi Eric, I believe that hyperslabs in xdmf are not working based on this gitlab issue:
https://gitlab.kitware.com/paraview/paraview/-/issues/19608
It is not clear to me what is happening with the XDMF format as I don’t think it is being actively maintained, note the website you referenced has not been touched since 2018, I have been trying to steer my people away from using XDMF because of this (frequently unsuccessfully).