It appears you have found a bug.
My guess is that you created your FreqPlot by choosing the Right-Click-
>Compute-> option on the stripchart. When this method is used to
create the plot it appears that the envelope information is not be
processed properly.
I've made note of it in our tracking system for review.
As Jennifer already mentioned you can work around this by generating a
new FreqPlot from the display builder and then dropping your parameter
into it.
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