Never had an issue with my WSM, but the radiant versus convection sounds logical. I also don't use water in the pan on the WSM, have used the sand mod and the clay pot mod. Don't know if either of those could be an issue. Suppose you could cook on the top grate and remove the heat sink and see if the plateu goes away.
Neil -- "Easy Bake Master"
NB Bandera [gone to heaven...conveniently located in Long Island, NY]
WSM A, WSM B, UDS, Easy Bake Oven
Pork Pistols BBQ
MOINK...It's what's for linner.