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I think this is because this is an even power.

sage: (-1)^(1/3)
(-1)^(1/3)

People complain about this all the time (including me, in the past) esp. because of plotting, though apparently this is consistent with Mma and Maple. But really it's the internal simplification in Ginac/Pynac that is doing this. Maxima, incidentally, does NOT do this.

(%i1) domain:complex;
(%o1)                               complex
(%i2) (-1)^(1/3);
                                        1/3
(%o2)                              (- 1)
(%i3) (-1)^(2/3);
                                        2/3
(%o3)                              (- 1)

I wonder what Mma and Maple do for this.