Well, this issue is due to the hex position / offset of the graphic. Unless you take an existing (and working as intended) wall graphic and copy the exact offsets and everything, you will have a hard time fixing this, as it's simply how the engine draws the light on the hex grid.
If it's only about fixed light sources, you can try to balance it out with adding additional light sources that only have a light range of 1 hex. You can also adjust the intensity, etc. But that's just faking the issue away for that one location.
/Edit: Also check that N/S and E/W flags are set correctly in the wall proto.