It's not a bug, it's a feature.
All the bakers (except "uv to svg") apply diffusion outside the uv islands. This is wanted because it reduces seams artifacts due to linear interpolation of texels across the border of uv islands on all mipmap levels. It also allows an elegant solution to "fill holes" where the raytracing process may have missed the high poly
geometry. Another desirable side effect is that it reduces compression artifacts and make the file smaller when applying jpeg compression on bitmaps.
Another (slower) solution that other baking software (eg XNormal) use to do that is to use dilation instead of diffusion (that means replicating the pixels around the border of uv islands instead of having them smoothly fade out).
"UV to svg" does dilation and not diffusion because there is no way to create diffusion color gradients in SVG, and dilation is cheap to do on vector graphics.
Some users have reported that they would prefer dilation over diffusion, but I suspect this is a question of being more used to it : I have yet to see a compelling example where dilation gives better visual results than diffusion when the texture is used on a mesh. We may offer a choice of diffusion vs dilation or a combination of both in the future.
