Maple agrees with magma here and says that the genus is one. Moreover, it computes a parametrization of the curve
sage: P=(((v + 2)*x^10 + (v - 2))/x^5, ((1/2*v - 3/2))/x^3)
The problem seems to be in singular.
sage: I = Ideal(p)
sage: I.genus()
10
If one looks at the documentation, p.geometric_genus?? it is said that it only works for prime fields. The documentation could be clearer for genus and I think that the method should raise an exception instead of giving a wrong answer.