Bug? Polynomial variable name matters

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The following calculation of an intersection of curves over QQbar raises an error within Singular on Sage v9.2.

P.<a, y, z> = ProjectiveSpace(QQbar, 2)
f = a^3 + z^3
C = Curve(f)
g = (a + y)^3 + z^3
D = Curve(g)
print(C.intersection_points(D))

It does work when replacing the variable name a by x.

I assume this is a bug but could not trace it down to the internals of rational_points and the Singular interface. The variables band d work, whereas c raises a different error. Does anyone know more about that? Is it already fixed in the newer versions? Should I post this as a ticket on https://trac.sagemath.org?