Thanks a lot @dokken, I was about to work on my code (but had a disk failure, making me losing today’s time).
I really wish to have a MWE with mixed elements, using a mesh from dolfinx rather than gmsh, I did try (see the bottom of my post at Unable to compute the curl of a vector field) but I didn’t finish. It’s not trivial for me to do a MWE with the features I want to conserve (i.e. ME space, and using my coupled PDE’s on a square mesh with 2 Dirichlet b.c. for “temp” (the temperature variable I solve for), and Neumann b.c.s for the voltage. The hard part for me is the mesh.