Hello,
I think there is a bug in the Camera Distorsion map.
(*) I created, procedurally, a simple Distorsion map by adding two Gradient Ramp maps with CoronaMix (Mix Operation=add). Everything is ok (no artifacts)
(*) Later, I changed the Tile parameter in both Gradient maps. As you can see, now, there are some artifacts on the borders.
I created the same Distortion map in Photoshop and the result was the same.