You are making a whole lot of assumptions on the ordering of the mesh vertices->dofs here.
Please see: I/O from XDMF/HDF5 files in dolfin-x - #32 by dokken
What you probably could do would be to read in a vertex marker in DOLFINx based on the function data (xdmf.read_meshtags) and map that input to a CG-1 function.