Surely this is a manifestation of the problem described in:
Antialiasing option behaves weird (polygon edges visible in ContourPlot)
Therefore, you should try using Style[plot, Antialiasing -> False] or other methods to disable anti-aliasing.
Somehow I had missed a closely related question which Jens directed me to. I believe it is a solution to this problem:
How can all those tiny polygons generated by RegionPlot be joined into a single FilledCurve?