Question: arctan and the csgn function


I was wondering why when I want to equate

arctan(0, z)

Maple returns

arctan(0, z)

I know that the maple arctan(x,y) (or atan2) plots from Pi/..-Pi/2, but at x = 0, depending on the sign of y, it should either return Pi/2 or -Pi/2. but the equation returned wouldnt do that, It will plot only from Pi/2 to 0. see below.

plot(arctan(0,z), z = -1..1)

first, why does maple evaluate it to that particular expression of csgn?
second, how to get a "correct" result?

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