The problem with negative capacitances is that magic does generate them, as corrections for overlaps between subcells (e.g., subcell A has 1fF, subcell B has 1fF, but they completely overlap, so the actual capacitance is 1fF, not 2fF. Since there is already 1fF in each extracted subcell, the solution is to add -1fF capacitance in the parent cell to compensate). That said, I think magic is doing an incorrect calculation somewhere and it would be helpful to get a minimalist example to debug.