Note that the n in question, the inwards pointing normal, from a mesh perspective is not continuous.
You are using the facet normal, which is a piecewise constant vector per facet.
Similarly, the gradient grad(T) is not continuous at element boundaries.
Therefore, I would suggest evaluating the field at the midpoint of each facet, where the field is continuous and well defined.
You can for instance do that with the functions in:
specifically; move_to_facet_quadrature. You can choose the number of points by controlling the degree of the quadrature space on the interface.