Three SVG files identical in every way but their colour have been rendered differently:
The issue is along the centre of the image from top to bottom. The black image has a barely-noticeable seam, and the red image has no seam at all, yet the green image has a very noticeable seam all the way through. It may be helpful to zoom in on the rasters to see what I'm talking about.
The left half of the image is mirrored to create the right half, which explains but doesn't excuse the seam - the two halves actually *overlap* each other!
Given that the seam comes and goes depending on the size of the raster, I'd tentatively put this down to rounding errors...