I adopted a less slick process to test if my students' series are p-series that avoids calling D so much. So while I think there is an issue here about D modifying the MathObject that called it, it's not an urgent matter for me at least. Something to look at some time.
WeBWorK Problems
Bug(s) (or user error?) with D derivative method
This forum has a limit to the number of forum postings you can make in a given time period - this is currently set at 5 posting(s) in 1 day