Thanks Gokul (and thanks Dhiman!). Clearly you are both correct and the plot is wrong, I am not sure what I was thinking when I wrote my first reply!!! This looks like a bug to me for iterations and the results calculation of the OFDM component. You can work around it by simulating a single iteration. Do not use a sweep. I, however, understand this is not a great fix when you want to simulate multiple power levels. Hopefully, we will get a fix for this soon.
Thanks again