It is not a bug, just a feedback. I start to use black for a project with math formula and I think black sometimes makes the code less readable.

Two simple examples:

Equations not fitting on one line

if True:
    if True:
        PK1_fft = np.real(
            + vx_fft.conj() * fx_fft
            + vy_fft.conj() * fy_fft
            + vz_fft.conj() * fz_fft
        )

is transformed by black -l 82 into

if True:
    if True:
        PK1_fft = np.real(
            +vx_fft.conj()
            * fx_fft
            + vy_fft.conj()
            * fy_fft
            + vz_fft.conj()
            * fz_fft
        )

The result is clearly less readable. It would be nice that black keeps the code as it is in such cases.

Spaces around pow operator

result = 2 * x**2 + 3 * x**(2/3)

is transformed by black into

result = 2 * x ** 2 + 3 * x ** (2 / 3)

which is less readable.

It seems to me it would make sense to allow "no space around **" because of the high precedence of the exponentiation (https://docs.python.org/3/reference/expressions.html#operator-precedence).