[dune-pdelab] Unphysical solution

[conf86 at web.de]

I have the following problem:
A sperical body has a radially dependent initial temperature u_0 from a shock wave as given in u_0.png. There is no further heat source and the body simply cools down.
So I have a*\partial_t u - \Delta u = 0 and Neumann boundary conditions for the outflow (black body radiation).

However as seen in u_t.png the solution looks very unphysical, because the heat should not distribute on the surface in this fashion.
I did a model calculation with a constant initial temperature beforehand and it looked ok, so I really don't know what was going wrong here.
However the gradient of the initial condition is rather high, could this be the reason?
I double-checked possible sign errors and similar, but I couldn't find any.

Do you have any idea what could be reason for this or where to look at?
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