This is a comment rather than an answer, but it grew too long for the comment box, and I wanted to show graphics, so I'll leave it here in hopes that someone better versed in the inner workings of MMA might explain this behavior. I am using your definition of the wrapper function pp.
First of all, I get a slightly different version of the "wrong" function:
Having said that, it seems that this behavior has something to do with the order of evaluation. In fact, if I pass the function to plot to the pp wrapper unevaluated, then I obtain the same well-behaved output as your "free standing" ParametricPlot:
pp[Unevaluated@ZernikeR[100, 0, r]]
The opposite is also true: if I pre-evaluate the ZernikeR function, then try to plot the result, I obtain the same result I had from feeding this function straight to your wrapper:
ZernikeR[100, 0, r];
ParametricPlot[{r, %}, {r, -1, 1}, PlotRange -> {Automatic, 1.05 {-1, 1}}]
I am afraid that this is as far as I was able to go though.