plotting - ContourPlot has extra mesh in PDF

When I create a contour plot it looks fine. When I copy as PDF, and paste into another document (e.g. Apple Mail, Keynote, etc.) it has an extra unwanted white mesh. This occurs even when the option Method -> {"TransparentPolygonMesh" -> True} is used.

Here is an example.

plot = ContourPlot[Exp[-Norm[{x, y}]^2], {x, -1, 1}, {y, -1, 1}, 
  Method -> {"TransparentPolygonMesh" -> True}]

Here is the picture that results, after copying as PDF, pasting into Mail, and doing a screen grab.

Here are the options:

Options[plot]


{DisplayFunction -> Identity, 
     AspectRatio -> 1, DisplayFunction :> Identity, Frame -> True, 
     FrameTicks -> {{Automatic, Automatic}, {Automatic, Automatic}}, 
     GridLinesStyle -> Directive[

    GrayLevel[0.5, 0.4]], 
     Method -> {"TransparentPolygonMesh" -> True, 
       "DefaultBoundaryStyle" -> Automatic}, 
     PlotRange -> {{-1, 1}, {-1, 1}}, PlotRangeClipping -> True, 
     PlotRangePadding -> {{Scaled[0.02], Scaled[0.02]}, {Scaled[0.02], 
        Scaled[0.02]}}, Ticks -> {Automatic, Automatic}}

I am using version 11.0.0.0 on Apple OS X 10.11.6 but the problem was present in previous versions.

I am aware that similar questions have been asked, but none provided a solution.