When using Ward with a glossy reflection lower than .75, there is a weird linear “scratch” highlight running diagonally across the lower right side of the sphere in the material editor.
Why does this happen?
It is the way Ward calculations work. 
They are cut off on some point.
Okay. Well I’m glad this is intended behavior. But can you explain it a bit more -- what do you mean by “they are cut off on some point”?
The formula for Ward cuts the reflections, when they get big enough. This is one of the reasons to have had red dots in scenes with Ward material.