APS have a few problems here and there with the laws of physics (and chemistry).
I said heat and TIME. If you understand a little of how tetra ethyl lead suppresses knock you will see where time comes into it.
At what mixture setting is cylinder head temperature at its highest?
That is the wrong question. The right question is "Under what conditions are unburnt gases exposed to the most heat for the longest time?"
The answer is: when combustion is slow, because you have heat from the already burnt gases acting on unburnt gases. Very lean mixture makes it worse because uneven mixing can give you very lean spots that are very slow to burn.
Cylinder head and valve temperature can contribute, but they are far from the only factor.