I observed the same thing when I increased the size of the mesh, e.g.
from dolfinx import mesh, fem
msh = mesh.create_unit_square(MPI.COMM_WORLD, 2, 2)
FE = FiniteElement("Lagrange", msh.ufl_cell(), 1)
ME = MixedElement([FE, FE])
V_ME = fem.FunctionSpace(msh, ME)
_, dofmaps = V_ME.sub(0).collapse()
print(dofmaps)
It returns [0, 1, 2, 6, 8, 10, 12, 14, 16]. Except for the first triangle element, other DOFs are ordered.
I am wondering if the feature of not-entirely ordered dofmap in a mixed element could cause interpolation issue especially across two non-matching meshes.