Question: Nonzero square of Grassmann number?

Consider the following:

Setup(anticommutativeprefix = psi):
psi^2,psi__1^2,psi__a^2;   # double underscores

Why does the square of psi__a (double underscore) not vanish as well? Or, perhaps, more to the point: why is psi__a not as well considered Grassmann-odd by Maple? Is this counter-intuitive behaviour intentional, or is it a bug?

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